Process for graphene foam-protected anode active materials for lithium batteries

ABSTRACT

A lithium-ion battery anode layer, comprising an anode active material embedded in pores of a solid graphene foam composed of multiple pores and pore walls, wherein (a) the pore walls contain a pristine graphene material having essentially no (less than 0.01%) non-carbon elements or a non-pristine graphene material having 0.01% to 5% by weight of non-carbon elements; (b) the anode active material is in an amount from 0.5% to 95% by weight based on the total weight of the graphene foam and the anode active material combined, and (c) some of the multiple pores are lodged with particles of the anode active material and other pores are particle-free, and the graphene foam is sufficiently elastic to accommodate volume expansion and shrinkage of the particles of the anode active material during a battery charge-discharge cycle to avoid expansion of the anode layer. Preferably, the solid graphene foam has a density from 0.01 to 1.7 g/cm 3 , a specific surface area from 50 to 2,000 m 2 /g, a thermal conductivity of at least 100 W/mK per unit of specific gravity, and/or an electrical conductivity no less than 1,000 S/cm per unit of specific gravity.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application of U.S. patentapplication Ser. No. 04/121,151, filed on Aug. 7, 2014, the contents ofwhich are incorporated by reference herein, in their entirety, for allpurposes.

FIELD OF THE INVENTION

The present invention relates generally to the field of rechargeablelithium battery and, more particularly, to the anode layer containing anew group of graphene foam-protected anode active materials and theprocess for producing same.

BACKGROUND OF THE INVENTION

The discussion of prior art information is herein divided into threeparts in this Background section: (a) a discussion on high-capacityanode active materials for lithium-ion batteries and long-standingissues associated with these materials; (b) the new 2-D nano materialcalled “graphene” and its prior use as a conductive substrate materialfor the anode active material; and (c) graphene-based foamed materialcalled “graphene foam”.

A unit cell or building block of a lithium-ion battery is typicallycomposed of an anode active material layer, an anode or negativeelectrode layer (containing an anode active material responsible forstoring lithium therein, a conductive additive, and a resin binder), anelectrolyte and porous separator, a cathode or positive electrode layer(containing a cathode active material responsible for storing lithiumtherein, a conductive additive, and a resin binder), and a separatecathode current collector. The electrolyte is in ionic contact with boththe anode active material and the cathode active material. A porousseparator is not required if the electrolyte is a solid-stateelectrolyte.

The binder in the binder layer is used to bond the anode active material(e.g. graphite or Si particles) and a conductive filler (e.g. carbonblack or carbon nanotube) together to form an anode layer of structuralintegrity, and to bond the anode layer to a separate anode currentcollector, which acts to collect electrons from the anode activematerial when the battery is discharged. In other words, in the negativeelectrode side of the battery, there are typically four differentmaterials involved: an anode active material, a conductive additive, aresin binder (e.g. polyvinylidine fluoride, PVDF, or styrene-butadienerubber, SBR), and an anode current collector (typically a sheet of Cufoil).

The most commonly used anode active materials for lithium-ion batteriesare natural graphite and synthetic graphite (or artificial graphite)that can be intercalated with lithium and the resulting graphiteintercalation compound (GIC) may be expressed as Li_(x)C₆, where x istypically less than 1. The maximum amount of lithium that can bereversibly intercalated into the interstices between graphene planes ofa perfect graphite crystal corresponds to x=1, defining a theoreticalspecific capacity of 372 mAh/g.

Graphite or carbon anodes can have a long cycle life due to the presenceof a protective solid-electrolyte interface layer (SEI), which resultsfrom the reaction between lithium and the electrolyte (or betweenlithium and the anode surface/edge atoms or functional groups) duringthe first several charge-discharge cycles. The lithium in this reactioncomes from some of the lithium ions originally intended for the chargetransfer purpose. As the SEI is formed, the lithium ions become part ofthe inert SEI layer and become irreversible, i.e. these positive ionscan no longer be shuttled back and forth between the anode and thecathode during charges/discharges. Therefore, it is desirable to use aminimum amount of lithium for the formation of an effective SEI layer.In addition to SEI formation, the irreversible capacity loss Q_(ir) canalso be attributed to graphite exfoliation caused by electrolyte/solventco-intercalation and other side reactions.

In addition to carbon- or graphite-based anode materials, otherinorganic materials that have been evaluated for potential anodeapplications include metal oxides, metal nitrides, metal sulfides, andthe like, and a range of metals, metal alloys, and intermetalliccompounds that can accommodate lithium atoms/ions or react with lithium.Among these materials, lithium alloys having a composition formula ofLi_(a)A (A is a metal or semiconductor element, such as Al and Si, and“a” satisfies 0<a≤5) are of great interest due to their high theoreticalcapacity, e.g., Li₄Si (3,829 mAh/g), Li₄₄Si (4,200 mAh/g), Li_(4.4)Ge(1,623 mAh/g), Li_(4.4)Sn (993 mAh/g), Li₃Cd (715 mAh/g), Li₃Sb (660mAh/g), Li_(4.4)Pb (569 mAh/g), LiZn (410 mAh/g), and Li₃Bi (385 mAh/g).However, as schematically illustrated in FIG. 1(A), in an anode composedof these high-capacity materials, severe pulverization (fragmentation ofthe alloy particles) occurs during the charge and discharge cycles dueto severe expansion and contraction of the anode active materialparticles induced by the insertion and extraction of the lithium ions inand out of these particles. The expansion and contraction, and theresulting pulverization, of active material particles, lead to loss ofcontacts between active material particles and conductive additives andloss of contacts between the anode active material and its currentcollector. This degradation phenomenon is illustrated in FIG. 1. Theseadverse effects result in a significantly shortened charge-dischargecycle life.

To overcome the problems associated with such mechanical degradation,three technical approaches have been proposed:

-   (1) reducing the size of the active material particle, presumably    for the purpose of reducing the total strain energy that can be    stored in a particle, which is a driving force for crack formation    in the particle. However, a reduced particle size implies a higher    surface area available for potentially reacting with the liquid    electrolyte to form a higher amount of SEI. Such a reaction is    undesirable since it is a source of irreversible capacity loss.-   (2) depositing the electrode active material in a thin film form    directly onto a current collector, such as a copper foil. However,    such a thin film structure with an extremely small    thickness-direction dimension (typically much smaller than 500 nm,    often necessarily thinner than 100 nm) implies that only a small    amount of active material can be incorporated in an electrode (given    the same electrode or current collector surface area), providing a    low total lithium storage capacity and low lithium storage capacity    per unit electrode surface area (even though the capacity per unit    mass can be large). Such a thin film must have a thickness less than    100 nm to be more resistant to cycling-induced cracking, further    diminishing the total lithium storage capacity and the lithium    storage capacity per unit electrode surface area. Such a thin-film    battery has very limited scope of application. A desirable and    typical electrode thickness is from 100 μm to 200 μm. These    thin-film electrodes (with a thickness of <500 nm or even <100 nm)    fall short of the required thickness by three (3) orders of    magnitude, not just by a factor of 3.-   (3) using a composite composed of small electrode active particles    protected by (dispersed in or encapsulated by) a less active or    non-active matrix, e.g., carbon-coated Si particles, sol gel    graphite-protected Si, metal oxide-coated Si or Sn, and    monomer-coated Sn nano particles. Presumably, the protective matrix    provides a cushioning effect for particle expansion or shrinkage,    and prevents the electrolyte from contacting and reacting with the    electrode active material. Examples of anode active particles are    Si, Sn, and SnO₂. Unfortunately, when an active material particle,    such as Si particle, expands (e.g. up to a volume expansion >300%)    during the battery charge step, the protective coating is easily    broken due to the mechanical weakness and/o brittleness of the    protective coating materials. There has been no high-strength and    high-toughness material available that is itself also lithium ion    conductive.

It may be further noted that the coating or matrix materials used toprotect active particles (such as Si and Sn) are carbon, sol gelgraphite, metal oxide, monomer, ceramic, and lithium oxide. Theseprotective materials are all very brittle, weak (of low strength),and/or non-conducting (e.g., ceramic or oxide coating). Ideally, theprotective material should meet the following requirements: (a) Thecoating or matrix material should be of high strength and stiffness sothat it can help to refrain the electrode active material particles,when lithiated, from expanding to an excessive extent. (b) Theprotective material should also have high fracture toughness or highresistance to crack formation to avoid disintegration during repeatedcycling. (c) The protective material must be inert (inactive) withrespect to the electrolyte, but be a good lithium ion conductor. (d) Theprotective material must not provide any significant amount of defectsites that irreversibly trap lithium ions. (e) The protective materialmust be lithium ion-conducting as well as electron-conducting. The priorart protective materials all fall short of these requirements. Hence, itwas not surprising to observe that the resulting anode typically shows areversible specific capacity much lower than expected. In many cases,the first-cycle efficiency is extremely low (mostly lower than 80% andsome even lower than 60%). Furthermore, in most cases, the electrode wasnot capable of operating for a large number of cycles. Additionally,most of these electrodes are not high-rate capable, exhibitingunacceptably low capacity at a high discharge rate.

Due to these and other reasons, most of prior art composite electrodeshave deficiencies in some ways, e.g., in most cases, less thansatisfactory reversible capacity, poor cycling stability, highirreversible capacity, ineffectiveness in reducing the internal stressor strain during the lithium ion insertion and extraction steps, andother undesirable side effects.

Complex composite particles of particular interest are a mixture ofseparate Si and graphite particles dispersed in a carbon matrix; e.g.those prepared by Mao, et al. [“Carbon-coated Silicon Particle Powder asthe Anode Material for Lithium Batteries and the Method of Making theSame,” US 2005/0136330 (Jun. 23, 2005)]. Also of interest are carbonmatrix-containing complex nano Si (protected by oxide) and graphiteparticles dispersed therein, and carbon-coated Si particles distributedon a surface of graphite particles Again, these complex compositeparticles led to a low specific capacity or for up to a small number ofcycles only. It appears that carbon by itself is relatively weak andbrittle and the presence of micron-sized graphite particles does notimprove the mechanical integrity of carbon since graphite particles arethemselves relatively weak. Graphite was used in these cases presumablyfor the purpose of improving the electrical conductivity of the anodematerial. Furthermore, polymeric carbon, amorphous carbon, orpre-graphitic carbon may have too many lithium-trapping sites thatirreversibly capture lithium during the first few cycles, resulting inexcessive irreversibility.

In summary, the prior art has not demonstrated a composite material thathas all or most of the properties desired for use as an anode materialin a lithium-ion battery. Thus, there is an urgent and continuing needfor a new anode for the lithium-ion battery that has a high cycle life,high reversible capacity, low irreversible capacity, small particlesizes (for high-rate capacity), and compatibility with commonly usedelectrolytes. There is also a need for a method of readily or easilyproducing such a material in large quantities.

Bulk natural graphite is a 3-D graphitic material with each graphiteparticle being composed of multiple grains (a grain being a graphitesingle crystal or crystallite) with grain boundaries (amorphous ordefect zones) demarcating neighboring graphite single crystals. Eachgrain is composed of multiple graphene planes that are oriented parallelto one another. A graphene plane in a graphite crystallite is composedof carbon atoms occupying a two-dimensional, hexagonal lattice. In agiven grain or single crystal, the graphene planes are stacked andbonded via van der Waal forces in the crystallographic c-direction(perpendicular to the graphene plane or basal plane). Although all thegraphene planes in one grain are parallel to one another, typically thegraphene planes in one grain and the graphene planes in an adjacentgrain are inclined at different orientations. In other words, theorientations of the various grains in a graphite particle typicallydiffer from one grain to another.

The constituent graphene planes of a graphite crystallite in a naturalor artificial graphite particle can be exfoliated and extracted orisolated to obtain individual graphene sheets of carbon atoms providedthe inter-planar van der Waals forces can be overcome. An isolated,individual graphene sheet of carbon atoms is commonly referred to assingle-layer graphene. A stack of multiple graphene planes bondedthrough van der Waals forces in the thickness direction with aninter-graphene plane spacing of approximately 0.3354 nm is commonlyreferred to as a multi-layer graphene. A multi-layer graphene platelethas up to 300 layers of graphene planes (<100 nm in thickness), but moretypically up to 30 graphene planes (<10 nm in thickness), even moretypically up to 20 graphene planes (<7 nm in thickness), and mosttypically up to 10 graphene planes (commonly referred to as few-layergraphene in scientific community). Single-layer graphene and multi-layergraphene sheets are collectively called “nano graphene platelets”(NGPs). Graphene or graphene oxide sheets/platelets (collectively, NGPs)are a new class of carbon nano material (a 2-D nano carbon) that isdistinct from the 0-D fullerene, the 1-D CNT, and the 3-D graphite.

Our research group pioneered the development of graphene materials andrelated production processes as early as 2002: (1) B. Z. Jang and W. C.Huang, “Nano-scaled Graphene Plates,” U.S. Pat. No. 7,071,258 (Jul. 4,2006), application submitted on Oct. 21, 2002; (2) B. Z. Jang, et al.“Process for Producing Nano-scaled Graphene Plates,” U.S. patentapplication Ser. No. 10/858,814 (Jun. 3, 2004); and (3) B. Z. Jang, A.Zhamu, and J. Guo, “Process for Producing Nano-scaled Platelets andNanocomposites,” U.S. patent application Ser. No. 11/509,424 (Aug. 25,2006).

In one process, graphene materials are obtained by intercalating naturalgraphite particles with a strong acid and/or an oxidizing agent toobtain a graphite intercalation compound (GIC) or graphite oxide (GO),as illustrated in FIG. 2(A) (process flow chart) and FIG. 2(B)(schematic drawing). The presence of chemical species or functionalgroups in the interstitial spaces between graphene planes serves toincrease the inter-graphene spacing (d₀₀₂, as determined by X-raydiffraction), thereby significantly reducing the van der Waals forcesthat otherwise hold graphene planes together along the c-axis direction.The GIC or GO is most often produced by immersing natural graphitepowder (20 in FIG. 1(A) and 100 in FIG. 1(B)) in a mixture of sulfuricacid, nitric acid (an oxidizing agent), and another oxidizing agent(e.g. potassium permanganate or sodium perchlorate). The resulting GIC(22 or 102) is actually some type of graphite oxide (GO) particles if anoxidizing agent is present during the intercalation procedure. This GICor GO is then repeatedly washed and rinsed in water to remove excessacids, resulting in a graphite oxide suspension or dispersion, whichcontains discrete and visually discernible graphite oxide particlesdispersed in water. In order to produce graphene materials, one canfollow one of the two processing routes after this rinsing step, brieflydescribed below:

Route 1 involves removing water from the suspension to obtain“expandable graphite,” which is essentially a mass of dried GIC or driedgraphite oxide particles. Upon exposure of expandable graphite to atemperature in the range of typically 800-1,050° C. for approximately 30seconds to 2 minutes, the GIC undergoes a rapid volume expansion by afactor of 30-300 to form “graphite worms” (24 or 104), which are each acollection of exfoliated, but largely un-separated graphite flakes thatremain interconnected. A SEM image of graphite worms is presented inFIG. 2.

In Route 1A, these graphite worms (exfoliated graphite or “networks ofinterconnected/non-separated graphite flakes”) can be re-compressed toobtain flexible graphite sheets or foils (26 or 106) that typically havea thickness in the range of 0.1 mm (100 μm)-0.5 mm (500 μm).Alternatively, one may choose to use a low-intensity air mill orshearing machine to simply break up the graphite worms for the purposeof producing the so-called “expanded graphite flakes” (49 or 108) whichcontain mostly graphite flakes or platelets thicker than 100 nm (hence,not a nano material by definition).

In Route 1B, the exfoliated graphite is subjected to high-intensitymechanical shearing (e.g. using an ultrasonicator, high-shear mixer,high-intensity air jet mill, or high-energy ball mill) to form separatedsingle-layer and multi-layer graphene sheets (collectively called NGPs,33 or 112), as disclosed in our U.S. application Ser. No. 10/858,814.Single-layer graphene can be as thin as 0.34 nm, while multi-layergraphene can have a thickness up to 100 nm, but more typically less than10 nm (commonly referred to as few-layer graphene). Multiple graphenesheets or platelets may be made into a sheet of NGP paper (34) using apaper-making process.

Route 2 entails ultrasonicating the graphite oxide suspension for thepurpose of separating/isolating individual graphene oxide sheets fromgraphite oxide particles. This is based on the notion that theinter-graphene plane separation has been increased from 0.3354 nm innatural graphite to 0.6-1.1 nm in highly oxidized graphite oxide,significantly weakening the van der Waals forces that hold neighboringplanes together. Ultrasonic power can be sufficient to further separategraphene plane sheets to form separated, isolated, or discrete grapheneoxide (GO) sheets. These graphene oxide sheets can then be chemically orthermally reduced to obtain “reduced graphene oxides” (RGO) typicallyhaving an oxygen content of 0.001%-10% by weight, more typically0.01%-5% by weight, most typically and preferably less than 2% byweight.

For the purpose of defining the claims of the instant application, NGPsor graphene materials include discrete sheets/platelets of single-layerand multi-layer (typically less than 10 layers) pristine graphene,graphene oxide, reduced graphene oxide (RGO), graphene fluoride,graphene chloride, graphene bromide, graphene iodide, hydrogenatedgraphene, nitrogenated graphene, chemically functionalized graphene,doped graphene (e.g. doped by B or N). Pristine graphene has essentially0% oxygen. RGO typically has an oxygen content of 0.001%-5% by weight.Graphene oxide (including RGO) can have 0.001%-50% by weight of oxygen.Other than pristine graphene, all the graphene materials have 0.001%-50%by weight of non-carbon elements (e.g. O, H, N, B, F, Cl, Br, I, etc.).These materials are herein referred to as non-pristine graphenematerials.

Another process for producing graphene, in a thin film form (typically<2 nm in thickness), is the catalytic chemical vapor deposition process.This catalytic CVD involves catalytic decomposition of hydrocarbon gas(e.g. C₂H₄) on Ni or Cu surface to form single-layer or few-layergraphene. With Ni or Cu being the catalyst, carbon atoms obtained viadecomposition of hydrocarbon gas molecules at a temperature of800-1,000° C. are directly deposited onto Cu foil surface orprecipitated out to the surface of a Ni foil from a Ni—C solid solutionstate to form a sheet of single-layer or few-layer graphene (less than 5layers). The Ni- or Cu-catalyzed CVD process does not lend itself to thedeposition of more than 5 graphene planes (typically <2 nm) beyond whichthe underlying Ni or Cu layer can no longer provide any catalyticeffect. The CVD graphene films are extremely expensive.

Our research group also pioneered the application of graphene materialsfor battery applications: One of our earlier applications discloses anano-scaled graphene platelet-based composite composition for use as alithium ion battery anode [A. Zhamu and B. Z. Jang, “Nano GraphenePlatelet-Based Composite Anode Compositions for Lithium Ion Batteries,”U.S. patent application Ser. No. 11/982,672 (Nov. 5, 2007); Now U.S.Pat. No. 7,745,047 (Jun. 29, 2010)]. This composition comprises: (a)micron- or nanometer-scaled particles or coating of an anode activematerial; and (b) a plurality of nano-scaled graphene platelets (NGPs),wherein a platelet comprises a graphene sheet or a stack of graphenesheets having a platelet thickness less than 100 nm and wherein theparticles or coating are physically attached or chemically bonded toNGPs. Nano graphene platelets (NGPs) are individual graphene sheets(individual basal planes of carbon atoms isolated from a graphitecrystal) or stacks of multiple graphene planes bonded together in thethickness direction. The NGPs have a thickness less than 100 nm and alength, width, or diameter that can be greater or less than 10 μm. Thethickness is more preferably less than 10 nm and most preferably lessthan 1 nm.

Disclosed in another patent application of ours is a more specificcomposition, which is composed of a 3-D network of NGPs and/or otherconductive filaments and anode active material particles that are bondedto these NGPs or filaments through a conductive binder [Jinjun Shi,Aruna Zhamu and Bor Z. Jang, “Conductive Nanocomposite-based Electrodesfor Lithium Batteries,” U.S. patent application Ser. No. 12/156,644(Jun. 4, 2008)]. Yet another application provides a nanographene-reinforced nanocomposite solid particle composition containingNGPs and electrode active material particles, which are both dispersedin a protective matrix (e.g. a carbon matrix) [Aruna Zhamu, Bor Z. Jang,and Jinjun Shi, “Nano Graphene Reinforced Nanocomposite for LithiumBattery Electrodes,” U.S. patent application Ser. No. 12/315,555 (Dec.4, 2008)].

After our discovery of graphene providing an outstanding support foranode active materials, many subsequent studies by others have confirmedthe effectiveness of this approach. For instance, Wang, et al.investigated self-assembled TiO₂-graphene hybrid nanostructures forenhanced Li-ion insertion [D. Wang, et al. “Self-Assembled TiO₂-GrapheneHybrid Nanostructures for Enhanced Li-Ion Insertion.” ACS Nano, 3 (2009)907-914]. The results indicate that, as compared with the pure TiO₂phase, the specific capacity of the hybrid was more than doubled at highcharge rates. The improved capacity at a high charge-discharge rate wasattributed to increased electrode conductivity afforded by a percolatedgraphene network embedded into the metal oxide electrodes. However, allthese earlier studies were focused solely on providing a network ofelectron-conducting paths for the anode active material particles andfailed to address other critical issues, such as ease of anode materialprocessing, electrode processability, electrode tap density (the abilityto pack a dense mass into a given volume), stability ofsolid-electrolyte interface (SEI), and long-term cycling stability. Forinstance, the method of preparing self-assembled hybrid nanostructuresis not amenable to mass production. These are all critically importantissues that must be addressed in a real battery manufacturingenvironment.

The present invention goes beyond and above these prior art efforts ofusing solid graphene sheets or platelets (NGPs) to form a 3-D conductivenetwork to support an anode active material. Specifically, the instantapplication makes use of a graphene foam material to protect the anodeactive material, by providing several other unexpected functions, inaddition to forming a 3-D conducting network. Hence, a brief discussionon the production of graphene foams should be helpful to the reader.

Generally speaking, a foam (or foamed material) is composed of pores (orcells) and pore walls (the solid portion of a foam material). The porescan be interconnected to form an open-cell foam. A graphene foam iscomposed of pores and pore walls that contain a graphene material. Thereare three major methods of producing graphene foams:

The first method is the hydrothermal reduction of graphene oxidehydrogel that typically involves sealing graphene oxide (GO) aqueoussuspension in a high-pressure autoclave and heating the GO suspensionunder a high pressure (tens or hundreds of atm) at a temperaturetypically in the range of 180-300° C. for an extended period of time(typically 12-36 hours). A useful reference for this method is givenhere: Y. Xu, et al. “Self-Assembled Graphene Hydrogel via a One-StepHydrothermal Process,” ACS Nano 2010, 4, 4324-4330. There are severalmajor issues associated with this method: (a) The high pressurerequirement makes it an impractical method for industrial-scaleproduction. For one thing, this process cannot be conducted on acontinuous basis. (b) It is difficult, if not impossible, to exercisecontrol over the pore size and the porosity level of the resultingporous structure. (c) There is no flexibility in terms of varying theshape and size of the resulting reduced graphene oxide (RGO) material(e.g. it cannot be made into a film shape). (d) The method involves theuse of an ultra-low concentration of GO suspended in water (e.g. 2mg/mL=2 g/L=2 kg/kL). With the removal of non-carbon elements (up to50%), one can only produce less than 2 kg of graphene material (RGO) per1000-liter suspension. Furthermore, it is practically impossible tooperate a 1000-liter reactor that has to withstand the conditions of ahigh temperature and a high pressure. Clearly, this is not a scalableprocess for mass production of porous graphene structures.

The second method is based on a template-assisted catalytic CVD process,which involves CVD deposition of graphene on a sacrificial template(e.g. Ni foam). The graphene material conforms to the shape anddimensions of the Ni foam structure. The Ni foam is then etched awayusing an etching agent, leaving behind a monolith of graphene skeletonthat is essentially an open-cell foam. A useful reference for thismethod is given here: Zongping Chen, et al., “Three-dimensional flexibleand conductive interconnected graphene networks grown by chemical vapourdeposition,” Nature Materials, 10 (June 2011) 424-428. There are severalproblems associated with such a process: (a) the catalytic CVD isintrinsically a very slow, highly energy-intensive, and expensiveprocess; (b) the etching agent is typically a highly undesirablechemical and the resulting Ni-containing etching solution is a source ofpollution. It is very difficult and expensive to recover or recycle thedissolved Ni metal from the etchant solution. (c) It is challenging tomaintain the shape and dimensions of the graphene foam without damagingthe cell walls when the Ni foam is being etched away. The resultinggraphene foam is typically very brittle and fragile. (d) The transportof the CVD precursor gas (e.g. hydrocarbon) into the interior of a metalfoam can be difficult, resulting in a non-uniform structure, sincecertain spots inside the sacrificial metal foam may not be accessible tothe CVD precursor gas. 0 This method does not lend itself to embeddinganode active material particles therein.

The third method of producing graphene foam also makes use of asacrificial material (e.g. colloidal polystyrene particles, PS) that iscoated with graphene oxide sheets using a self-assembly approach. Forinstance, Choi, et al. prepared chemically modified graphene (CMG) paperin two steps: fabrication of free-standing PS/CMG films by vacuumfiltration of a mixed aqueous colloidal suspension of CMG and PS (2.0 μmPS spheres), followed by removal of PS beads to generate 3D macro-pores.[B. G. Choi, et al., “3D Macroporous Graphene Frameworks forSupercapacitors with High Energy and Power Densities,” ACS Nano, 6(2012) 4020-4028.] Choi, et al. fabricated well-ordered free-standingPS/CMG paper by filtration, which began with separately preparing anegatively charged CMG colloidal and a positively charged PS suspension.A mixture of CMG colloidal and PS suspension was dispersed in solutionunder controlled pH (=2), where the two compounds had the same surfacecharges (zeta potential values of +13±2.4 mV for CMG and +68±5.6 mV forPS). When the pH was raised to 6, CMGs (zeta potential=−29±3.7 mV) andPS spheres (zeta potential=+51±2.5 mV) were assembled due to theelectrostatic interactions and hydrophobic characteristics between them,and these were subsequently integrated into PS/CMG composite paperthrough a filtering process. This method also has several shortcomings:(a) This method requires very tedious chemical treatments of bothgraphene oxide and PS particles. (b) The removal of PS by toluene alsoleads to weakened macro-porous structures. (c) Toluene is a highlyregulated chemical and must be treated with extreme caution. (d) Thepore sizes are typically excessively big (e.g. several μm), too big formany useful applications.

The above discussion clearly indicates that every prior art method orprocess for producing graphene foams has major deficiencies. Further,none of the earlier work makes use of graphene foam as a protectivematerial for an anode active material of a lithium battery.

Thus, it is an object of the present invention to provide acost-effective process for producing highly conductive, mechanicallyrobust graphene foams in large quantities. This graphene foam alsocontains anode active material particles residing in the pores of thisfoam and being protected by this foam. This process does not involve theuse of an environmentally unfriendly chemical. This process enables theflexible design and control of the porosity level and pore sizes.

It is another object of the present invention to provide a process forproducing graphene foam-protected anode active material wherein thegraphene foam exhibits a thermal conductivity, electrical conductivity,elastic modulus, and/or compressive strength that is comparable to orgreater than those of the graphite/carbon foams. The internal pores ofthe protective graphene foam expands and shrinks congruently with theexpansion and shrinkage of the embedded anode active material particles,enabling long-term cycling stability of a lithium battery featuring ahigh-capacity anode active material (such as Si, Sn, SnO₂, Co₃O₄, etc.).

It is another object of the present invention to provide an anode layerthat exhibits a combination of exceptional thermal conductivity,electrical conductivity, mechanical strength, and elastic modulusunmatched by any anode layer commonly used in a lithium-ion battery.

Yet another object of the present invention is to provide a graphenefoam-protected anode active material wherein the graphene foam isselected from (a) a pristine graphene foam that contains essentially allcarbon only and preferably have a pore size range from 2 nm to 200 nm;or (b) non-pristine graphene foams (graphene fluoride, graphenechloride, nitrogenated graphene, etc.) that contains at least 0.001% byweight (typically from 0.01% to 5% by weight and most typically from0.01% to 2%) of non-carbon elements.

SUMMARY OF THE INVENTION

Herein reported is a process for producing a significantly improvedanode layer that provides not only a robust 3-D network ofelectron-conducting paths and high conductivity, but also enables theanode material to be readily made into an electrode layer with a highelectrode tap density, a sufficiently large electrode thickness(typically 50-300 μm to ensure a sufficient amount of output current), alarge weight percentage of anode active material (with respect to thetotal amount of the non-active materials, such as conductive additiveand binder, in an electrode and a separate current collector combined),and long-term cycling stability. Both the reversible capacity and thefirst-cycle efficiency are also significantly improved over those ofstate-of-the-art anode materials.

Briefly, the present invention provides a new anode layer compositionwherein an anode active material (e.g. Si, Li, or SnO₂ particles) isnaturally lodged in pores of a graphene foam that is beyond just havingan adequate room to accommodate expansion of the anode active material.The presently invented graphene foam also exhibits a unique “elastic”property in that the cell walls (solid portion of the foam) can becompressed to tightly embrace anode active material particles when ananode layer is made. When individual particles expand (upon Liintercalation), the volume expansion is accommodated by local cellwalls, without inducing a volume change of the entire anode layer(hence, not exerting internal pressure to the battery). During thesubsequent discharge cycle, these particles shrink; yet the local cellwalls shrink or snap back in a congruent manner, maintaining a goodcontact between cell walls and the particles (remaining capable ofaccepting Li⁺ ions and electrons during the next charge cycle).

The invented anode or negative electrode layer comprises an anode activematerial embedded in pores of a solid graphene foam composed of multiplepores and pore walls, wherein (a) the pore walls contain a pristinegraphene material having essentially zero % of non-carbon elements or anon-pristine graphene material having 0.001% to 5% by weight ofnon-carbon elements, wherein the non-pristine graphene is selected fromgraphene oxide, reduced graphene oxide, graphene fluoride, graphenechloride, graphene bromide, graphene iodide, hydrogenated graphene,nitrogenated graphene, boron-doped graphene, nitrogen-doped graphene,chemically functionalized graphene, or a combination thereof; (b) theanode active material is in an amount from 0.5% to 99% by weight basedon the total weight of the graphene foam and the anode active materialcombined; and (c) some pores are lodged with the particles of the anodeactive material and other pores are particle-free, and the graphene foamis sufficiently elastic to accommodate volume expansion and shrinkage ofthe particles of the anode active material during a batterycharge-discharge cycle to avoid an expansion of the anode layer.

The solid graphene foam typically has a density from 0.01 to 1.7 g/cm³,a specific surface area from 50 to 2,000 m²/g, a thermal conductivity ofat least 100 W/mK per unit of specific gravity, and/or an electricalconductivity no less than 1,000 S/cm per unit of specific gravity.

In an embodiment, the anode active material is selected from the groupconsisting of: (a) silicon (Si), germanium (Ge), tin (Sn), lead (Pb),antimony (Sb), bismuth (Bi), zinc (Zn), aluminum (Al), titanium (Ti),nickel (Ni), cobalt (Co), and cadmium (Cd); (b) alloys or intermetalliccompounds of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Ti, Ni, Co, or Cd withother elements; (c) oxides, carbides, nitrides, sulfides, phosphides,selenides, and tellurides of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Ti, Fe, Ni,Co, V, or Cd, and their mixtures, composites, or lithium-containingcomposites; (d) salts and hydroxides of Sn; (e) lithium titanate,lithium manganate, lithium aluminate, lithium-containing titanium oxide,lithium transition metal oxide; (f) prelithiated versions thereof (g)particles of Li, Li alloy, or surface-stabilized Li; and (h)combinations thereof. The prelithiated version of a high-capacity anodeactive material means an anode active material that is intercalated orinserted with a desired amount of lithium before this anode activematerial is introduced into the foam pores, or before this anode activematerial is mixed with the graphene material to form a foamed structure.In a preferred embodiment, the anode active material contains aprelithiated Si, prelithiated Ge, prelithiated Sn, prelithiated SnO_(x),prelithiated SiO_(x), prelithiated iron oxide, prelithiated VO₂,prelithiated Co₃O₄, prelithiated Ni₃O₄, or a combination thereof,wherein x=1 to 2.

Preferably, the anode active material is in a form of nano particle,nano wire, nano fiber, nano tube, nano sheet, nano belt, nano ribbon, ornano coating having a thickness or diameter less than 100 nm. Morepreferably, the anode active material has a dimension less than 20 nm.

In a preferred embodiment, the anode layer further comprises a carbon orgraphite material therein, wherein the carbon or graphite material is inelectronic contact with or deposited onto the anode active material.Most preferably, this carbon or graphite material embraces the particlesof the anode active material and the embraced particles are then lodgedin the pores of the graphene foam. The carbon or graphite material maybe selected from polymeric carbon, amorphous carbon, chemical vapordeposition carbon, coal tar pitch, petroleum pitch, meso-phase pitch,carbon black, coke, acetylene black, activated carbon, fine expandedgraphite particle with a dimension smaller than 100 nm, artificialgraphite particle, natural graphite particle, or a combination thereof.Most preferably, the anode layer further comprises a conductiveprotective coating, selected from a carbon material, electronicallyconductive polymer, conductive metal oxide, conductive metal coating, ora lithium-conducting material, which is deposited onto or wrapped aroundthe nano particle, nano wire, nano fiber, nano tube, nano sheet, nanobelt, nano ribbon, or nano coating. Preferably, the nano particle, nanowire, nano fiber, nano tube, nano sheet, nano belt, nano ribbon, or nanocoating is prelithiated. The coating can be a lithium-conductingmaterial.

Typically, in the invented anode layer, the pore walls contain stackedgraphene planes having an inter-plane spacing d₀₀₂ from 0.3354 nm to0.36 nm as measured by X-ray diffraction. The pore walls can contain apristine graphene and the solid graphene foam has a density from 0.5 to1.7 g/cm³ or the pores have a pore size from 2 nm to 200 nm, preferablyfrom 2 nm to 100 nm. Alternatively, the non-pristine graphene materialcontains a content of non-carbon elements from 0.01% to 2.0% by weight.In one embodiment, the pore walls contain graphene fluoride and thesolid graphene foam contains a fluorine content from 0.01% to 2.0% byweight. In another embodiment, the pore walls contain graphene oxide andthe solid graphene foam contains an oxygen content from 0.01% to 2.0% byweight. Typically, the non-carbon elements include an element selectedfrom oxygen, fluorine, chlorine, bromine, iodine, nitrogen, hydrogen, orboron Typically, the solid graphene foam has a specific surface areafrom 200 to 2,000 m²/g or a density from 0.1 to 1.5 g/cm³.

In a preferred embodiment, the anode layer is made from a layer that isa continuous-length roll sheet form having a thickness no greater than300 μm and a length of at least 2 meters and is produced by aroll-to-roll process.

In a desired embodiment, the graphene foam in the anode layer has anoxygen content or non-carbon content less than 1% by weight, and thepore walls have an inter-graphene spacing less than 0.35 nm, a thermalconductivity of at least 250 W/mK per unit of specific gravity, and/oran electrical conductivity no less than 2,500 S/cm per unit of specificgravity.

In a preferred embodiment, the graphene foam has an oxygen content ornon-carbon content less than 0.01% by weight and the pore walls containstacked graphene planes having an inter-graphene spacing less than 0.34nm, a thermal conductivity of at least 300 W/mK per unit of specificgravity, and/or an electrical conductivity no less than 3,000 S/cm perunit of specific gravity. Further preferably, the graphene foam has anoxygen content or non-carbon content no greater than 0.01% by weight andthe pore walls contain stacked graphene planes having an inter-graphenespacing less than 0.336 nm, a mosaic spread value no greater than 0.7, athermal conductivity of at least 350 W/mK per unit of specific gravity,and/or an electrical conductivity no less than 3,500 S/cm per unit ofspecific gravity. Most preferably, the graphene foam has pore wallscontaining stacked graphene planes having an inter-graphene spacing lessthan 0.336 nm, a mosaic spread value no greater than 0.4, a thermalconductivity greater than 400 W/mK per unit of specific gravity, and/oran electrical conductivity greater than 4,000 S/cm per unit of specificgravity.

The pore walls may contain stacked graphene planes having aninter-graphene spacing less than 0.337 nm and a mosaic spread value lessthan 1.0. In an embodiment, the solid graphene foam exhibits a degree ofgraphitization no less than 80% and/or a mosaic spread value less than0.4. More preferably, the solid graphene foam exhibits a degree ofgraphitization no less than 90% and/or a mosaic spread value no greaterthan 0.4. Typically, in the invented anode layer, the pore walls containa 3D network of interconnected graphene planes. The graphene foamcontains pores having a pore size from 20 nm to 500 nm.

The present invention also provides a lithium battery containing theanode or negative electrode as defined above, a cathode or positiveelectrode, and an electrolyte in ionic contact with the anode and thecathode. This lithium battery can further contain a cathode currentcollector in electronic contact with the cathode.

In an embodiment, the lithium battery further contains an anode currentcollector in electronic contact with the anode. Alternatively and morepreferably, in the lithium battery, the graphene foam operates as ananode current collector to collect electrons from the anode activematerial during a charge of the lithium battery, which contains noseparate or additional current collector. The lithium battery can be alithium-ion battery, lithium metal battery, lithium-sulfur battery, orlithium-air battery.

In a preferred embodiment, the solid graphene foam-protected anodeactive material is made into a continuous-length roll sheet form (a rollof a continuous foam sheet) having a thickness no greater than 200 μmand a length of at least 1 meter long, preferably at least 2 meters,further preferably at least 10 meters, and most preferably at least 100meters. This sheet roll is produced by a roll-to-roll process. There hasbeen no prior art graphene foam that is made into a sheet roll form. Ithas not been previously found or suggested possible to have aroll-to-roll process for producing a continuous length of graphene foam,either pristine or non-pristine.

The presently invented anode layer may be produced by a processcomprising:

-   (a) preparing a graphene dispersion having particles of an anode    active material and a graphene material dispersed in a liquid    medium, wherein the graphene material is selected from pristine    graphene, graphene oxide, reduced graphene oxide, graphene fluoride,    graphene chloride, graphene bromide, graphene iodide, hydrogenated    graphene, nitrogenated graphene, chemically functionalized graphene,    or a combination thereof and wherein the dispersion contains an    optional blowing agent;-   (b) dispensing and depositing the graphene dispersion onto a surface    of a supporting substrate (e.g. plastic film, rubber sheet, metal    foil, glass sheet, paper sheet, etc.) to form a wet layer of    graphene-anode material mixture, wherein the dispensing and    depositing procedure includes subjecting the graphene dispersion to    an orientation-inducing stress;-   (c) partially or completely removing the liquid medium from the wet    layer of graphene-anode material mixture to form a dried layer of    material mixture having a content of non-carbon elements (e.g. O, H,    N, B, F, Cl, Br, I, etc.) no less than 5% by weight; and-   (d) heat treating the dried layer of material mixture at a first    heat treatment temperature from 100° C. to 3,200° C. at a desired    heating rate sufficient to induce volatile gas molecules from the    non-carbon elements or to activate the blowing agent for producing    the anode layer.

The solid graphene foam in the anode layer typically has a density from0.01 to 1.7 g/cm³ (more typically from 0.1 to 1.5 g/cm³, and even moretypically from 0.1 to 1.0 g/cm³, and most typically from 0.2 to 0.75g/cm³), or a specific surface area from 50 to 3,000 m²/g (more typicallyfrom 200 to 2,000 m²/g, and most typically from 500 to 1,500 m²/g).

This optional blowing agent is not required if the graphene material hasa content of non-carbon elements (e.g. O, H, N, B, F, Cl, Br, I, etc.)no less than 5% by weight (preferably no less than 10%, furtherpreferably no less than 20%, even more preferably no less than 30% or40%, and most preferably up to 50%). The subsequent high temperaturetreatment serves to remove a majority of these non-carbon elements fromthe graphene material, generating volatile gas species that producepores or cells in the solid graphene material structure. In other words,quite surprisingly, these non-carbon elements play the role of a blowingagent. Hence, an externally added blowing agent is optional (notrequired). However, the use of a blowing agent can provide addedflexibility in regulating or adjusting the porosity level and pore sizesfor a desired application. The blowing agent is typically required ifthe non-carbon element content is less than 5%, such as pristinegraphene that is essentially all-carbon.

The blowing agent can be a physical blowing agent, a chemical blowingagent, a mixture thereof, a dissolution-and-leaching agent, or amechanically introduced blowing agent.

The process may further include a step of heat-treating the anode layerat a second heat treatment temperature higher than the first heattreatment temperature for a length of time sufficient for obtaining ananode layer wherein the pore walls contain stacked graphene planeshaving an inter-plane spacing d₀₀₂ from 0.3354 nm to 0.40 nm and acontent of non-carbon elements less than 5% by weight (typically from0.001% to 2%). When the resulting non-carbon element content is from0.1% to 2.0%, the inter-plane spacing d₀₀₂ is typically from 0.337 nm to0.40 nm.

If the original graphene material in the dispersion contains a fractionof non-carbon elements higher than 5% by weight, the graphene materialin the solid graphene foam (after the heat treatment) containsstructural defects that are induced during the step (d) of heattreating. The liquid medium can be simply water and/or an alcohol, whichis environmentally benign.

In a preferred embodiment, the process is a roll-to-roll process whereinsteps (b) and (c) include feeding the supporting substrate from a feederroller to a deposition zone, continuously or intermittently depositingthe graphene dispersion onto a surface of the supporting substrate toform the wet layer thereon, drying the wet layer to form the dried layerof material mixture, and collecting the dried layer of material mixturedeposited on the supporting substrate on a collector roller. Such aroll-to-roll or reel-to-reel process is a truly industrial-scale,massive manufacturing process that can be automated.

In one embodiment, the first heat treatment temperature is from 100° C.to 1,500° C. In another embodiment, the second heat treatmenttemperature includes at least a temperature selected from (A) 300-1,500°C., (B) 1,500-2,100° C., and/or (C) 2,100-3,200° C. In a specificembodiment, the second heat treatment temperature includes a temperaturein the range of 300-1,500° C. for at least 1 hour and then a temperaturein the range of 1,500-3,200° C. for at least 1 hour.

There are several surprising results of conducting first and/or secondheat treatments to the dried graphene-anode active material mixturelayer, and different heat treatment temperature ranges enable us toachieve different purposes, such as (a) removal of non-carbon elementsfrom the graphene material (e.g. thermal reduction of fluorinatedgraphene to obtain graphene or reduced graphene fluoride, RGF)) whichgenerate volatile gases to produce pores or cells in a graphenematerial, (b) activation of the chemical or physical blowing agent toproduce pores or cells, (c) chemical merging or linking of graphenesheets to significantly increase the lateral dimension of graphenesheets in the foam walls (solid portion of the foam), (d) healing ofdefects created during fluorination, oxidation, or nitrogenation ofgraphene planes in a graphite particle, and (e) re-organization andperfection of graphitic domains or graphite crystals. These differentpurposes or functions are achieved to different extents within differenttemperature ranges. The non-carbon elements typically include an elementselected from oxygen, fluorine, chlorine, bromine, iodine, nitrogen,hydrogen, or boron. Quite surprisingly, even under low-temperaturefoaming conditions, heat-treating induces chemical linking, merging, orchemical bonding between graphene sheets, often in an edge-to-edgemanner (some in face-to-face manner).

In one embodiment, the solid graphene foam, minus the anode activematerial, has a specific surface area from 200 to 2,000 m²/g. In oneembodiment, the solid graphene foam has a density from 0.1 to 1.5 g/cm³.In an embodiment, step (d) of heat treating the dried layer ofgraphene-anode active material mixture at a first heat treatmenttemperature is conducted under a compressive stress. In anotherembodiment, the process comprises a compression step to reduce athickness, pore size, or porosity level of the sheet of graphene foam.In battery cells, the anode layer typically has a thickness from 10 μmto 300 μm, more typically from 50 μm to 200 μm.

In an embodiment, the graphene dispersion has at least 3% by weight ofgraphene oxide dispersed in the liquid medium to form a liquid crystalphase. In another embodiment, the graphene dispersion contains agraphene oxide dispersion prepared by immersing a graphitic material ina powder or fibrous form in an oxidizing liquid in a reaction vessel ata reaction temperature for a length of time sufficient to obtain thegraphene dispersion wherein the graphitic material is selected fromnatural graphite, artificial graphite, meso-phase carbon, meso-phasepitch, meso-carbon micro-bead, soft carbon, hard carbon, coke, carbonfiber, carbon nano-fiber, carbon nano-tube, or a combination thereof andwherein the graphene oxide has an oxygen content no less than 5% byweight.

In an embodiment, the first heat treatment temperature contains atemperature in the range of 80° C.-300° C. and, as a result, thegraphene foam has an oxygen content or non-carbon element content lessthan 5%, and the pore walls have an inter-graphene spacing less than0.40 nm, a thermal conductivity of at least 150 W/mK (more typically atleast 200 W/mk) per unit of specific gravity, and/or an electricalconductivity no less than 2,000 S/cm per unit of specific gravity.

In a preferred embodiment, the first and/or second heat treatmenttemperature contains a temperature in the range of 300° C.-1,500° C.and, as a result, the graphene foam has an oxygen content or non-carboncontent less than 1%, and the pore walls have an inter-graphene spacingless than 0.35 nm, a thermal conductivity of at least 250 W/mK per unitof specific gravity, and/or an electrical conductivity no less than2,500 S/cm per unit of specific gravity.

When the first and/or second heat treatment temperature contains atemperature in the range of 1,500° C.-2,100° C., the graphene foam hasan oxygen content or non-carbon content less than 0.01% and pore wallshave an inter-graphene spacing less than 0.34 nm, a thermal conductivityof at least 300 W/mK per unit of specific gravity, and/or an electricalconductivity no less than 3,000 S/cm per unit of specific gravity.

When the first and/or second heat treatment temperature contains atemperature greater than 2,100° C., the graphene foam has an oxygencontent or non-carbon content no greater than 0.001% and pore walls havean inter-graphene spacing less than 0.336 nm, a mosaic spread value nogreater than 0.7, a thermal conductivity of at least 350 W/mK per unitof specific gravity, and/or an electrical conductivity no less than3,500 S/cm per unit of specific gravity.

If the first and/or second heat treatment temperature contains atemperature no less than 2,500° C., the graphene foam has pore wallscontaining stacked graphene planes having an inter-graphene spacing lessthan 0.336 nm, a mosaic spread value no greater than 0.4, and a thermalconductivity greater than 400 W/mK per unit of specific gravity, and/oran electrical conductivity greater than 4,000 S/cm per unit of specificgravity.

In one embodiment, the pore walls contain stacked graphene planes havingan inter-graphene spacing less than 0.337 nm and a mosaic spread valueless than 1.0. In another embodiment, the solid wall portion of thegraphene foam exhibits a degree of graphitization no less than 80%and/or a mosaic spread value less than 0.4. In yet another embodiment,the solid wall portion of the graphene foam exhibits a degree ofgraphitization no less than 90% and/or a mosaic spread value no greaterthan 0.4.

Typically, the pore walls contain a 3D network of interconnectedgraphene planes that are electron-conducting pathways. The cell wallscontain graphitic domains or graphite crystals having a lateraldimension (L_(a), length or width) no less than 20 nm, more typicallyand preferably no less than 40 nm, still more typically and preferablyno less than 100 nm, still more typically and preferably no less than500 nm, often greater than 1 μm, and sometimes greater than 10 μm. Thegraphitic domains typically have a thickness from 1 nm to 200 nm, moretypically from 1 nm to 100 nm, further more typically from 1 nm to 40nm, and most typically from 1 nm to 30 nm.

Preferably, the solid graphene foam contains pores having a pore sizefrom 2 nm to 10 μm (preferably 2 nm to 500 nm and more preferably from 2nm to 200 nm). It may be noted that it has not been possible to useNi-catalyzed CVD to produce graphene foams having a pore size range of2-20 nm. This is due to the notion that it has not been proven possibleto prepare Ni foam templates having such a pore size range and notpossible for the hydrocarbon gas (precursor molecules) to readily enterNi foam pores of these sizes. These Ni foam pores must also beinterconnected. Additionally, the sacrificial plastic colloidal particleapproaches have resulted in macro-pores that are in the size range ofmicrons to millimeters.

In a preferred embodiment, the present invention provides a roll-to-rollprocess for producing an anode layer composed of an anode activematerial and a solid graphene foam, which is composed of multiple poresand pore walls. The process comprises: (a) preparing a graphenedispersion having an anode active material and a graphene materialdispersed in a liquid medium, wherein the dispersion optionally containsa blowing agent; (b) continuously or intermittently dispensing anddepositing the graphene dispersion onto a surface of a supportingsubstrate to form a wet layer of graphene-anode active material mixture,wherein the supporting substrate is a continuous thin film supplied froma feeder roller and collected on a collector roller; (c) partially orcompletely removing the liquid medium from the wet layer to form a driedlayer of material mixture; and (d) heat treating the dried layer ofmaterial mixture at a first heat treatment temperature from 100° C. to3,000° C. at a desired heating rate sufficient to activate the blowingagent for producing said solid graphene foam having a density from 0.01to 1.7 g/cm³ or a specific surface area from 50 to 3,000 m²/g.

The orientation-inducing stress may be a shear stress. As an example,the shear stress can be encountered in a situation as simple as a“doctor's blade” that guides the spreading of graphene dispersion over aplastic or glass surface during a manual casting process. As anotherexample, an effective orientation-inducing stress is created in anautomated roll-to-roll coating process in which a “knife-on-roll”configuration dispenses the graphene dispersion over a moving solidsubstrate, such as a plastic film. The relative motion between thismoving film and the coating knife acts to effect orientation of graphenesheets along the shear stress direction.

This orientation-inducing stress is a critically important step in theproduction of the presently invented graphene foams due to thesurprising observation that the shear stress enables the graphene sheetsto align along a particular direction (e.g. X-direction orlength-direction) to produce preferred orientations and facilitatecontacts between graphene sheets along foam walls. Further surprisingly,these preferred orientations and improved graphene-to-graphene contactsfacilitate chemical merging or linking between graphene sheets duringthe subsequent heat treatment of the dried graphene layer. Suchpreferred orientations and improved contacts are essential to theeventual attainment of exceptionally high thermal conductivity,electrical conductivity, elastic modulus, and mechanical strength of theresulting graphene foam. In general, these great properties could not beobtained without such a shear stress-induced orientation control.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(A) schematic illustrating the notion that expansion of Siparticles, upon lithium intercalation, can lead to pulverization of Siparticles, interruption of the conductive paths formed by the conductiveadditive, and loss of contact with the current collector;

FIG. 1(B) schematic of a graphene foam-protected anode active materialaccording to an embodiment.

FIG. 2(A) A flow chart illustrating various prior art processes ofproducing exfoliated graphite products (flexible graphite foils andexpanded graphite flakes), along with a process for producing pristinegraphene foam 40 a or graphene oxide foams 40 b;

FIG. 2(B) Schematic drawing illustrating the processes for producingconventional paper, mat, film, and membrane of simply aggregatedgraphite or NGP flakes/platelets. All processes begin with intercalationand/or oxidation treatment of graphitic materials (e.g. natural graphiteparticles).

FIG. 3(A) schematic of a prior art lithium-ion battery cell, wherein theanode layer is a thin coating of an anode active material itself; and

FIG. 3(B) schematic of another lithium-ion battery; the anode layerbeing composed of particles of an anode active material, a conductiveadditive (not shown) and a resin binder (not shown).

FIG. 4A possible mechanism of chemical linking between graphene oxidesheets, which mechanism effectively increases the graphene sheet lateraldimensions.

FIG. 5(A) Thermal conductivity values vs. specific gravity of the GOsuspension-derived foam produced by the presently invented process,meso-phase pitch-derived graphite foam, and Ni foam-template assistedCVD graphene foam;

FIG. 5(B) Thermal conductivity values of the GO suspension-derived foam,sacrificial plastic bead-templated GO foam, and the hydrothermallyreduced GO graphene foam;

FIG. 5(C) electrical conductivity data for the GO suspension-derivedfoam produced by the presently invented process and the hydrothermallyreduced GO graphene foam; and

FIG. 5(D) The specific capacity of a lithium battery having a pristinegraphene foam-protected Si and that of a pristine graphene-Si mixture asan electrode material (lithium metal as the counter-electrode in ahalf-cell configuration) plotted as a function of the number ofcharge-discharge cycles.

FIG. 6(A) Thermal conductivity values (vs. specific gravity values up to1.02 g/cm³) of the GO suspension-derived foam, meso-phase pitch-derivedgraphite foam, and Ni foam-template assisted CVD graphene foam;

FIG. 6(B) Thermal conductivity values of the GO suspension-derived foam,sacrificial plastic bead-templated GO foam, and hydrothermally reducedGO graphene foam (vs. specific gravity values up to 1.02 g/cm³);

FIG. 6(C) Specific capacities of three anode layers: the presentlyinvented GO-derived graphene foam-protected Sn, Sn only (withoutgraphene foam protection), graphene foam only, and theoreticalprediction based on the rule-of-mixture law;

FIG. 6(D) Specific capacities of two anode layers: the presentlyinvented GO-derived graphene foam-protected SnO₂ and hydrothermallyreduced GO-derived graphene foam-protected SnO₂.

FIG. 7 Thermal conductivity values of graphene foam samples derived fromGO and GF (graphene fluoride) as a function of the specific gravity.

FIG. 8 Thermal conductivity values of graphene foam samples derived fromGO and pristine graphene as a function of the final (maximum) heattreatment temperature.

FIG. 9(A) Inter-graphene plane spacing in graphene foam walls asmeasured by X-ray diffraction;

FIG. 9(B) the oxygen content in the GO suspension-derived graphene foam.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

This invention is directed at the anode layer (negative electrode layer)containing a high-capacity anode material for the lithium secondarybattery, which is preferably a secondary battery based on a non-aqueouselectrolyte, a polymer gel electrolyte, an ionic liquid electrolyte, aquasi-solid electrolyte, or a solid-state electrolyte. The shape of alithium secondary battery can be cylindrical, square, button-like, etc.The present invention is not limited to any battery shape orconfiguration. For convenience, we will use Si, Sn, or SnO₂ asillustrative examples of a high-capacity anode active material. Thisshould not be construed as limiting the scope of the invention.

As illustrated in FIG. 4(A) and FIG. 4(B), a lithium-ion battery cell istypically composed of an anode current collector (e.g. Cu foil), ananode or negative electrode (anode layer containing an anode activematerial, conductive additive, and binder), a porous separator and/or anelectrolyte component, a cathode electrode (cathode active material,conductive additive, and resin binder), and a cathode current collector(e.g. Al foil). In a more commonly used cell configuration (FIG. 4(B)),the anode layer is composed of particles of an anode active material(e.g. graphite, Sn, SnO₂, or Si), a conductive additive (e.g. carbonblack particles), and a resin binder (e.g. SBR or PVDF). This anodelayer is typically 50-300 μm thick (more typically 100-200 μm) to giverise to a sufficient amount of current per unit electrode area. Thisthickness range is an industry-accepted constraint under which a batterydesigner must work. This constraint is due to several reasons: (a) theexisting battery electrode coating machines are not equipped to coatexcessively thin or excessively thick electrode layers; (b) a thinnerlayer is preferred based on the consideration of reduced lithium iondiffusion path lengths; but, too thin a layer (e.g. <<100 μm) does notcontain a sufficient amount of an active lithium storage material(hence, insufficient current output); and (c) all non-active materiallayers in a battery cell (e.g. current collectors, conductive additive,binder resin, and separator) must be kept to a minimum in order toobtain a minimum overhead weight and a maximum lithium storagecapability and, hence, a maximized energy density (Wk/kg or Wh/L, ofcell).

In a less commonly used cell configuration, as illustrated in FIG. 4(A),the anode active material is deposited in a thin film form directly ontoan anode current collector, such as a sheet of copper foil. However,such a thin film structure with an extremely small thickness-directiondimension (typically much smaller than 500 nm, often necessarily thinnerthan 100 nm) implies that only a small amount of active material can beincorporated in an electrode (given the same electrode or currentcollector surface area), providing a low total lithium storage capacityand low lithium storage capacity per unit electrode surface area. Such athin film must have a thickness less than 100 nm to be more resistant tocycling-induced cracking. Such a constraint further diminishes the totallithium storage capacity and the lithium storage capacity per unitelectrode surface area. Such a thin-film battery has very limited scopeof application. On the other hand, a Si layer thicker than 100 nm hasbeen found to exhibit poor cracking resistance during batterycharge/discharge cycles. It takes but a few cycles to get such a thickfilm fragmented. A desirable electrode thickness is at least 100 μm.These thin-film electrodes (with a thickness <100 nm) fall short of therequired thickness by three (3) orders of magnitude. As a furtherproblem, Si or SiO₂ film-based anode layers cannot be too thick eithersince these materials are not conductive to both electrons and lithiumions. A large layer thickness implies an excessively high internalresistance.

In other words, there are several conflicting factors that must beconsidered concurrently when it comes to the design and selection of ananode active material in terms of material type, shape, size, porosity,and electrode layer thickness. Thus far, there has been no effectivesolution offered by any prior art teaching to these often conflictingproblems. We have solved these challenging issues that have troubledbattery designers and electrochemists alike for more than 30 years bydeveloping the graphene foam-protected anode active material.

The present invention provides an anode layer containing (A) a sheet ofsolid graphene foam composed of multiple pores and pore walls and (B) ananode active material with the particles of this anode active materialresiding in some of these pores; some pores remaining unoccupied, actingto cushion volume expansion of anode active material particles. Theinvention also provides a process for producing such an anode layer.

More specifically, the invented anode or negative electrode layercomprises an anode active material embedded in pores of a solid graphenefoam, which is composed of multiple pores and pore walls (solid portionof the graphene foam), wherein (a) the pore walls contain a pristinegraphene material having essentially zero % of non-carbon elements or anon-pristine graphene material having 0.001% to 5% by weight ofnon-carbon elements, wherein the non-pristine graphene is selected fromgraphene oxide, reduced graphene oxide, graphene fluoride, graphenechloride, graphene bromide, graphene iodide, hydrogenated graphene,nitrogenated graphene, boron-doped graphene, nitrogen-doped graphene,chemically functionalized graphene, or a combination thereof; (b) theanode active material is in an amount from 0.5% to 99% by weight(preferably from 2% to 90% by weight and more preferably from 5% to 80%by weight) based on the total weight of the graphene foam and the anodeactive material combined; and (c) some pores are lodged with theparticles of the anode active material and other pores areparticle-free, and the graphene foam is sufficiently elastic toaccommodate volume expansion and shrinkage of the particles of the anodeactive material during battery charge-discharge cycles to avoidexpansion of the anode layer. The bonded graphene planes in the foamwalls produced by the presently invented process are found to be capableof elastically deforming to the extent that is responsive to theexpansion and shrinkage of the anode active material particles.

The solid graphene foam typically has a density from 0.01 to 1.7 g/cm³,(more typically from 0.05 to 1.6 g/cm³, further more typically from 0.1to 1.5 g/cm³, and more desirably from 0.5 to 0.01 to 1.3 g/cm³), aspecific surface area from 50 to 2,000 m²/g, a thermal conductivity ofat least 100 W/mK per unit of specific gravity, and/or an electricalconductivity no less than 1,000 S/cm per unit of specific gravity. Itmay be noted that these ranges of physical densities are not arbitrarilyselected ranges. On the one hand, these densities are designed so thatthe internal pore amount (level of porosity) is sufficiently large toaccommodate the maximum expansion of an anode active material, whichvaries from one anode active material to another (e.g. approximately300%-380% maximum volume expansion for Si and approximately 200% forSnO₂). On the other hand, the pore amount cannot be too large (orphysical density being too low); otherwise, the pore walls of thegraphene foam structure cannot be sufficiently elastic (or, not capableof undergoing a large deformation that is fully recoverable orreversible).

Ideally, the pores should expand to the same extent as the embracedanode active material particle does; and should shrink back to the sameextent as the anode active material particle. In other words, thegraphene foam walls must be fully elastic to meet such a requirement.This is a most challenging task; but, we have surprisingly observed thatgood elasticity of graphene foam can be achieved with sufficientlylong/wide graphene planes (length/width of graphene planes larger thanpore diameters) and a sufficient amount (5%-50% of total pore volumes)of small pores (2-100 nm) that are not occupied by an anode activematerial particle.

The anode active material may be selected from the group consisting of:(a) silicon (Si), germanium (Ge), tin (Sn), lead (Pb), antimony (Sb),bismuth (Bi), zinc (Zn), aluminum (Al), titanium (Ti), nickel (Ni),cobalt (Co), and cadmium (Cd); (b) alloys or intermetallic compounds ofSi, Ge, Sn, Pb, Sb, Bi, Zn, Al, Ti, Ni, Co, or Cd with other elements;(c) oxides, carbides, nitrides, sulfides, phosphides, selenides, andtellurides of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Ti, Fe, Ni, Co, V, or Cd,and their mixtures, composites, or lithium-containing composites; (d)salts and hydroxides of Sn; (e) lithium titanate, lithium manganate,lithium aluminate, lithium-containing titanium oxide, lithium transitionmetal oxide; (f) prelithiated versions thereof; (g) particles of Li, Lialloy, or surface-stabilized Li; and (h) combinations thereof. Particlesof Li or Li alloy, particularly surface-stabilized Li particles (e.g.wax-coated Li particles), were found to be good anode active materialper se or an extra lithium source to compensate for the loss of Li ionsthat are otherwise supplied only from the cathode active material. Thepresence of these Li or Li-alloy particles was found to significantlyimprove the cycling performance of a lithium-ion cell.

The anode active material may include particles of natural graphite orartificial graphite, prelithiated or non-lithiated. The particles of theanode active material may be in the form of a nano particle, nano wire,nano fiber, nano tube, nano sheet, nano belt, nano ribbon, or nanocoating. Preferably, the nano particle, nano wire, nano fiber, nanotube, nano sheet, nano belt, nano ribbon, or nano coating isprelithiated. Preferably, the particles are embraced by anelectron-conducting and/or lithium-conducting coating, such as anamorphous carbon produced by chemical vapor deposition (CVD) orpyrolization of a resin.

Briefly, the process for producing the invented anode layer comprisesthe following steps:

(a) preparing a graphene dispersion having particles of an anode activematerial and sheets or molecules of a graphene material dispersed in aliquid medium, wherein the graphene material is selected from pristinegraphene, graphene oxide, reduced graphene oxide, graphene fluoride,graphene chloride, graphene bromide, graphene iodide, hydrogenatedgraphene, nitrogenated graphene, chemically functionalized graphene, ora combination thereof and wherein the dispersion contains an optionalblowing agent with a blowing agent-to-graphene material weight ratiofrom 0/1.0 to 1.0/1.0 (this blowing agent is normally required if thegraphene material is pristine graphene, typically having a blowingagent-to-pristine graphene weight ratio from 0.01/1.0 to 1.0/1.0);

(b) dispensing and depositing the graphene dispersion onto a surface ofa supporting substrate (e.g. plastic film, rubber sheet, metal foil,glass sheet, paper sheet, etc.) to form a wet layer of graphene-anodematerial mixture, wherein the dispensing and depositing procedure (e.g.coating or casting) preferably includes subjecting the graphenedispersion to an orientation-inducing stress;

(c) partially or completely removing the liquid medium from the wetlayer of graphene material to form a dried layer of material mixture,with the graphene material having a content of non-carbon elements (e.g.O, H, N, B, F, Cl, Br, I, etc.) no less than 5% by weight (thisnon-carbon content, when being removed via heat-induced decomposition,produces volatile gases that act as a foaming agent or blowing agent);and

(d) heat treating the dried layer of material mixture at a first heattreatment temperature from 100° C. to 3,000° C. at a desired heatingrate sufficient to induce volatile gas molecules from the non-carbonelements in the graphene material or to activate the blowing agent forproducing the solid graphene foam. The graphene foam typically has adensity from 0.01 to 1.7 g/cm³ (more typically from 0.1 to 1.5 g/cm³,and even more typically from 0.1 to 1.0 g/cm³, and most typically from0.2 to 0.75 g/cm³), or a specific surface area from 50 to 3,000 m²/g(more typically from 200 to 2,000 m²/g, and most typically from 500 to1,500 m²/g).

The pores in the graphene foam are formed slightly before, during, orafter sheets of a graphene material are (1) chemically linked/mergedtogether (edge-to-edge and/or face-to-face) typically at a temperaturefrom 100 to 1,500° C. and/or (2) re-organized into larger graphitecrystals or domains (herein referred to as re-graphitization) along thepore walls at a high temperature (typically >2,100° C. and moretypically >2,500° C.). It may be noted that the particles of the anodeactive material may be in the form of small particulate, wire, rod,sheet, platelet, ribbon, tube, etc. with a size of <20 μm (preferably<10 μm, more preferably <5 μm, further preferably <1 μm, still morepreferably <300 nm, and most preferably <100 nm). These particles arenaturally embraced by graphene sheets, typically leaving behind some gapbetween the particle and the embracing graphene sheets. Hence, whereparticles are present, there are pores in the graphene foam. However,there are additional pores that are formed due to the evolution ofvolatile gases (from a blowing agent and/or non-carbon elements, such as—OH, —F, etc.) during the heat treatment of the dried graphene layer.These pores play the role of cushioning the local volume expansion ofanode particles, thereby avoiding global expansion of the resultinganode layer. The ability of the pore walls to snap back according to theshrinkage extent of the anode particles comes from the surroundinggraphene sheets that are bonded and joint to form larger and strongergraphene planes during heat treatments.

A blowing agent or foaming agent is a substance which is capable ofproducing a cellular or foamed structure via a foaming process in avariety of materials that undergo hardening or phase transition, such aspolymers (plastics and rubbers), glass, and metals. They are typicallyapplied when the material being foamed is in a liquid state. It has notbeen previously known that a blowing agent can be used to create afoamed material while in a solid state. More significantly, it has notbeen taught or hinted that an aggregate of sheets of a graphene materialcan be converted into a graphene foam via a blowing agent. The cellularstructure in a matrix is typically created for the purpose of reducingdensity, increasing thermal resistance and acoustic insulation, whileincreasing the thickness and relative stiffness of the original polymer.

Blowing agents or related foaming mechanisms to create pores or cells(bubbles) in a matrix for producing a foamed or cellular material, canbe classified into the following groups:

-   -   (a) Physical blowing agents: e.g. hydrocarbons (e.g. pentane,        isopentane, cyclopentane), chlorofluorocarbons (CFCs),        hydrochlorofluorocarbons (HCFCs), and liquid CO₂. The        bubble/foam-producing process is endothermic, i.e. it needs heat        (e.g. from a melt process or the chemical exotherm due to        cross-linking), to volatize a liquid blowing agent.    -   (b) Chemical blowing agents: e.g. isocyanate, azo-, hydrazine        and other nitrogen-based materials (for thermoplastic and        elastomeric foams), sodium bicarbonate (e.g. baking soda, used        in thermoplastic foams). Here gaseous products and other        by-products are formed by a chemical reaction, promoted by        process or a reacting polymer's exothermic heat. Since the        blowing reaction involves forming low molecular weight compounds        that act as the blowing gas, additional exothermic heat is also        released. Powdered titanium hydride is used as a foaming agent        in the production of metal foams, as it decomposes to form        titanium and hydrogen gas at elevated temperatures.        Zirconium (II) hydride is used for the same purpose. Once formed        the low molecular weight compounds will never revert to the        original blowing agent(s), i.e. the reaction is irreversible.    -   (c) Mixed physical/chemical blowing agents: e.g. used to produce        flexible polyurethane (PU) foams with very low densities. Both        the chemical and physical blowing can be used in tandem to        balance each other out with respect to thermal energy        released/absorbed; hence, minimizing temperature rise. For        instance, isocyanate and water (which react to form CO₂) are        used in combination with liquid CO₂ (which boils to give gaseous        form) in the production of very low density flexible PU foams        for mattresses.    -   (d) Mechanically injected agents: Mechanically made foams        involve methods of introducing bubbles into liquid polymerizable        matrices (e.g. an unvulcanized elastomer in the form of a liquid        latex). Methods include whisking-in air or other gases or low        boiling volatile liquids in low viscosity lattices, or the        injection of a gas into an extruder barrel or a die, or into        injection molding barrels or nozzles and allowing the shear/mix        action of the screw to disperse the gas uniformly to form very        fine bubbles or a solution of gas in the melt. When the melt is        molded or extruded and the part is at atmospheric pressure, the        gas comes out of solution expanding the polymer melt immediately        before solidification.    -   (e) Soluble and leachable agents: Soluble fillers, e.g. solid        sodium chloride crystals mixed into a liquid urethane system,        which is then shaped into a solid polymer part, the sodium        chloride is later washed out by immersing the solid molded part        in water for some time, to leave small inter-connected holes in        relatively high density polymer products.    -   (f) We have found that the above five mechanisms can all be used        to create pores in the graphene materials while they are in a        solid state. Another mechanism of producing pores in a graphene        material is through the generation and vaporization of volatile        gases by removing those non-carbon elements in a        high-temperature environment. This is a unique self-foaming        process that has never been previously taught or suggested.

In a preferred embodiment, the graphene material in the dispersion isselected from pristine graphene, graphene oxide, reduced graphene oxide,graphene fluoride, graphene chloride, graphene bromide, graphene iodide,hydrogenated graphene, nitrogenated graphene, chemically functionalizedgraphene, or a combination thereof. The starting graphitic material forproducing any one of the above graphene materials may be selected fromnatural graphite, artificial graphite, meso-phase carbon, meso-phasepitch, meso-carbon micro-bead, soft carbon, hard carbon, coke, carbonfiber, carbon nano-fiber, carbon nano-tube, or a combination thereof.

For instance, as discussed in the Background section, the graphene oxide(GO) may be obtained by immersing powders or filaments of a startinggraphitic material (e.g. natural graphite powder) in an oxidizing liquidmedium (e.g. a mixture of sulfuric acid, nitric acid, and potassiumpermanganate) in a reaction vessel at a desired temperature for a periodof time (typically from 0.5 to 96 hours, depending upon the nature ofthe starting material and the type of oxidizing agent used). Theresulting graphite oxide particles may then be subjected to thermalexfoliation or ultrasonic wave-induced exfoliation to produce GO sheets.

Pristine graphene may be produced by direct ultrasonication (also knownas liquid phase production) or supercritical fluid exfoliation ofgraphite particles. These processes are well-known in the art. Multiplepristine graphene sheets may be dispersed in water or other liquidmedium with the assistance of a surfactant to form a suspension. Achemical blowing agent may then be dispersed into the dispersion (38 inFIG. 1(A)). This suspension is then cast or coated onto the surface of asolid substrate (e.g. glass sheet or Al foil). When heated to a desiredtemperature, the chemical blowing agent is activated or decomposed togenerate volatile gases (e.g. N₂ or CO₂), which act to form bubbles orpores in an otherwise mass of solid graphene sheets, forming a pristinegraphene foam 40 a.

Fluorinated graphene or graphene fluoride is herein used as an exampleof the halogenated graphene material group. There are two differentapproaches that have been followed to produce fluorinated graphene: (1)fluorination of pre-synthesized graphene: This approach entails treatinggraphene prepared by mechanical exfoliation or by CVD growth withfluorinating agent such as XeF₂, or F-based plasmas; (2) Exfoliation ofmultilayered graphite fluorides: Both mechanical exfoliation and liquidphase exfoliation of graphite fluoride can be readily accomplished [F.Karlicky, et al. “Halogenated Graphenes: Rapidly Growing Family ofGraphene Derivatives” ACS Nano, 2013, 7 (8), pp 6434-6464].

Interaction of F₂ with graphite at high temperature leads to covalentgraphite fluorides (CF)_(n) or (C₂F)_(n), while at low temperaturesgraphite intercalation compounds (GIC) C_(x)F (2≤x≤24) form. In (CF)_(n)carbon atoms are sp3-hybridized and thus the fluorocarbon layers arecorrugated consisting of trans-linked cyclohexane chairs. In (C₂F)_(n)only half of the C atoms are fluorinated and every pair of the adjacentcarbon sheets are linked together by covalent C—C bonds. Systematicstudies on the fluorination reaction showed that the resulting F/C ratiois largely dependent on the fluorination temperature, the partialpressure of the fluorine in the fluorinating gas, and physicalcharacteristics of the graphite precursor, including the degree ofgraphitization, particle size, and specific surface area. In addition tofluorine (F₂), other fluorinating agents may be used, although most ofthe available literature involves fluorination with F₂ gas, sometimes inpresence of fluorides.

For exfoliating a layered precursor material to the state of individuallayers or few-layers, it is necessary to overcome the attractive forcesbetween adjacent layers and to further stabilize the layers. This may beachieved by either covalent modification of the graphene surface byfunctional groups or by non-covalent modification using specificsolvents, surfactants, polymers, or donor-acceptor aromatic molecules.The process of liquid phase exfoliation includes ultrasonic treatment ofa graphite fluoride in a liquid medium.

The nitrogenation of graphene can be conducted by exposing a graphenematerial, such as graphene oxide, to ammonia at high temperatures(200-400° C.). Nitrogenated graphene could also be formed at lowertemperatures by a hydrothermal method; e.g. by sealing GO and ammonia inan autoclave and then increased the temperature to 150-250° C. Othermethods to synthesize nitrogen doped graphene include nitrogen plasmatreatment on graphene, arc-discharge between graphite electrodes in thepresence of ammonia, ammonolysis of graphene oxide under CVD conditions,and hydrothermal treatment of graphene oxide and urea at differenttemperatures.

The pore walls (cell walls or solid graphene portion) in the graphenefoam of the presently invented anode layer contain chemically bonded andmerged graphene planes. These planar aromatic molecules or grapheneplanes (hexagonal structured carbon atoms) are well interconnectedphysically and chemically. The lateral dimensions (length or width) ofthese planes are huge (e.g. from 20 nm to >10 μm), typically severaltimes or even orders of magnitude larger than the maximum crystallitedimension (or maximum constituent graphene plane dimension) of thestarting graphite particles. The graphene sheets or planes areessentially merged and/or interconnected to form electron-conductingpathways with low resistance. This is a unique and new class of materialthat has not been previously discovered, developed, or suggested topossibly exist.

In order to illustrate how the presently invented process works toproduce a graphene foam-protected anode layer, we herein make use ofgraphene oxide (GO) and graphene fluoride (GF) as two examples. Theseshould not be construed as limiting the scope of our claims. In eachcase, the first step involves preparation of a graphene dispersion (e.g.GO+water or GF+organic solvent, DMF) containing an optional blowingagent. If the graphene material is pristine graphene containing nonon-carbon elements, a blowing agent is required.

In step (b), the GF or GO suspension (21 in FIG. 1(A), but now alsocontaining particles of a desired anode active material) is formed intoa wet GF or GO layer 35 on a solid substrate surface (e.g. PET film orglass) preferably under the influence of a shear stress. One example ofsuch a shearing procedure is casting or coating a thin film of GF or GOsuspension using a coating machine. This procedure is similar to a layerof varnish, paint, coating, or ink being coated onto a solid substrate.The roller, “doctor's blade”, or wiper creates a shear stress when thefilm is shaped, or when there is a relative motion between theroller/blade/wiper and the supporting substrate. Quite unexpectedly andsignificantly, such a shearing action enables the planar GF or GO sheetsto well align along, for instance, a shearing direction. Furthersurprisingly, such a molecular alignment state or preferred orientationis not disrupted when the liquid components in the GF or GO suspensionare subsequently removed to form a well-packed layer of highly alignedGF or GO sheets that are at least partially dried. The dried GF or GOmass 37 a has a high birefringence coefficient between an in-planedirection and the normal-to-plane direction.

In an embodiment, this GF or GO layer, each containing an anode activematerial therein, is then subjected to a heat treatment to activate theblowing agent and/or the thermally-induced reactions that remove thenon-carbon elements (e.g. F, O, etc.) from the graphene sheets togenerate volatile gases as by-products. These volatile gases generatepores or bubbles inside the solid graphene material, pushing solidgraphene sheets into a foam wall structure, forming a graphene oxidefoam 40 b. If no blowing agent is added, the non-carbon elements in thegraphene material preferably occupy at least 10% by weight of thegraphene material (preferably at least 20%, and further preferably atleast 30%). The first (initial) heat treatment temperature is typicallygreater than 80° C., preferably greater than 100° C., more preferablygreater than 300° C., further more preferably greater than 500° C. andcan be as high as 1,500° C. The blowing agent is typically activated ata temperature from 80° C. to 300° C., but can be higher. The foamingprocedure (formation of pores, cells, or bubbles) is typically completedwithin the temperature range of 80-1,500° C. Quite surprisingly, thechemical linking or merging between graphene planes (GO or GF planes) inan edge-to-edge and face-to-face manner can occur at a relatively lowheat treatment temperature (e.g. even as low as from 150 to 300° C.).

The foamed graphene material may be subjected to a further heattreatment that involves at least a second temperature that issignificantly higher than the first heat treatment temperature.

A properly programmed heat treatment procedure can involve just a singleheat treatment temperature (e.g. a first heat treatment temperatureonly), at least two heat treatment temperatures (first temperature for aperiod of time and then raised to a second temperature and maintained atthis second temperature for another period of time), or any othercombination of heat treatment temperatures (HTT) that involve an initialtreatment temperature (first temperature) and a final HTT (second),higher than the first. The highest or final HTT that the dried graphenelayer experiences may be divided into four distinct HTT regimes:

-   Regime 1 (80° C. to 300° C.): In this temperature range (the thermal    reduction regime and also the activation regime for a blowing agent,    if present), a GO or GF layer primarily undergoes thermally-induced    reduction reactions, leading to a reduction of oxygen content or    fluorine content from typically 20-50% (of 0 in GO) or 10-25% (of F    in GF) to approximately 5-6%. This treatment results in a reduction    of inter-graphene spacing in foam walls from approximately 0.6-1.2    nm (as dried) down to approximately 0.4 nm, and an increase in    thermal conductivity to 200 W/mK per unit specific gravity and/or    electrical conductivity to 2,000 S/cm per unit of specific gravity.    (Since one can vary the level of porosity and, hence, specific    gravity of a graphene foam material and, given the same graphene    material, both the thermal conductivity and electric conductivity    values vary with the specific gravity, these property values must be    divided by the specific gravity to facilitate a fair comparison.)    Even with such a low temperature range, some chemical linking    between graphene sheets occurs. The inter-GO or inter-GF planar    spacing remains relatively large (0.4 nm or larger). Many O- or    F-containing functional groups survive.-   Regime 2 (300° C.-1,500° C.): In this chemical linking regime,    extensive chemical combination, polymerization, and cross-linking    between adjacent GO or GF sheets occur. The oxygen or fluorine    content is reduced to typically <1.0% (e.g. 0.7%) after chemical    linking, resulting in a reduction of inter-graphene spacing to    approximately 0.345 nm. This implies that some initial    re-graphitization has already begun at such a low temperature, in    stark contrast to conventional graphitizable materials (such as    carbonized polyimide film) that typically require a temperature as    high as 2,500° C. to initiate graphitization. This is another    distinct feature of the presently invented graphene foam and its    production processes. These chemical linking reactions result in an    increase in thermal conductivity to 250 W/mK per unit of specific    gravity, and/or electrical conductivity to 2,500-4,000 S/cm per unit    of specific gravity.-   Regime 3 (1,500-2,500° C.): In this ordering and re-graphitization    regime, extensive graphitization or graphene plane merging occurs,    leading to significantly improved degree of structural ordering in    the foam walls. As a result, the oxygen or fluorine content is    reduced to typically 0.01% and the inter-graphene spacing to    approximately 0.337 nm (achieving degree of graphitization from 1%    to approximately 80%, depending upon the actual HTT and length of    time). The improved degree of ordering is also reflected by an    increase in thermal conductivity to >350 W/mK per unit of specific    gravity, and/or electrical conductivity to >3,500 S/cm per unit of    specific gravity.-   Regime 4 (higher than 2,500° C.): In this re-crystallization and    perfection regime, extensive movement and elimination of grain    boundaries and other defects occur, resulting in the formation of    nearly perfect single crystals or poly-crystalline graphene crystals    with huge grains in the foam walls, which can be orders of magnitude    larger than the original grain sizes of the starting graphite    particles for the production of GO or GF. The oxygen or fluorine    content is essentially eliminated, typically 0%-0.001%. The    inter-graphene spacing is reduced to down to approximately 0.3354 nm    (degree of graphitization from 80% to nearly 100%), corresponding to    that of a perfect graphite single crystal. The foamed structure thus    obtained exhibits a thermal conductivity of >400 W/mK per unit of    specific gravity, and electrical conductivity of >4,000 S/cm per    unit of specific gravity.

The presently invented graphene foam structure containing an anodeactive material therein can be obtained by heat-treating the dried GO orGF layer with a temperature program that covers at least the firstregime (typically requiring 1-4 hours in this temperature range if thetemperature never exceeds 500° C.), more commonly covers the first tworegimes (1-2 hours preferred), still more commonly the first threeregimes (preferably 0.5-2.0 hours in Regime 3), and can cover all the 4regimes (including Regime 4 for 0.2 to 1 hour, may be implemented toachieve the highest conductivity).

The maximum HHT also depends on the type of anode active materialembraced by the graphene material. For instance, Sn (meltingpoint=231.9° C.) will not require a temperature higher than 300° C. andcannot tolerate a temperature higher than 500° C. Yet, tin dioxide(TiO₂), having a melting point of 1,630° C., can tolerate a temperatureup to 2,100° C.

If the graphene material is selected from the group of non-pristinegraphene materials consisting of graphene oxide, reduced graphene oxide,graphene fluoride, graphene chloride, graphene bromide, graphene iodide,hydrogenated graphene, nitrogenated graphene, chemically functionalizedgraphene, or a combination thereof, and wherein the maximum heattreatment temperature (e.g. both the first and second heat treatmenttemperatures) is (are) less than 2,500° C., then the resulting solidgraphene foam typically contains a content of non-carbon elements in therange of 0.01% to 2.0% by weight (non-pristine graphene foam).

X-ray diffraction patterns were obtained with an X-ray diffractometerequipped with CuKcv radiation. The shift and broadening of diffractionpeaks were calibrated using a silicon powder standard. The degree ofgraphitization, g, was calculated from the X-ray pattern using theMering's Eq, d₀₀₂=0.3354 g+0.344 (1−g), where d₀₀₂ is the interlayerspacing of graphite or graphene crystal in nm. This equation is validonly when d₀₀₂ is equal or less than approximately 0.3440 nm. Thegraphene foam walls having a d₀₀₂ higher than 0.3440 nm reflects thepresence of oxygen- or fluorine-containing functional groups (such as—F, —OH, >O, and —COOH on graphene molecular plane surfaces or edges)that act as a spacer to increase the inter-graphene spacing.

Another structural index that can be used to characterize the degree ofordering of the stacked and bonded graphene planes in the foam walls ofgraphene and conventional graphite crystals is the “mosaic spread,”which is expressed by the full width at half maximum of a rocking curve(X-ray diffraction intensity) of the (002) or (004) reflection. Thisdegree of ordering characterizes the graphite or graphene crystal size(or grain size), amounts of grain boundaries and other defects, and thedegree of preferred grain orientation. A nearly perfect single crystalof graphite is characterized by having a mosaic spread value of 0.2-0.4.Most of our graphene walls have a mosaic spread value in this range of0.2-0.4 (if produced with a heat treatment temperature (HTT) no lessthan 2,500° C.). However, some values are in the range of 0.4-0.7 if theHTT is between 1,500 and 2,500° C., and in the range of 0.7-1.0 if theHTT is between 300 and 1,500° C.

Illustrated in FIG. 4 is a plausible chemical linking mechanism whereonly 2 aligned GO molecules are shown as an example, although a largenumber of GO molecules can be chemically linked together to form a foamwall. Further, chemical linking could also occur face-to-face, not justedge-to-edge for GO, GF, and chemically functionalized graphene sheets.These linking and merging reactions proceed in such a manner that themolecules are chemically merged, linked, and integrated into one singleentity. The graphene sheets (GO or GF sheets) completely lose their ownoriginal identity and they no longer are discretesheets/platelets/flakes. The resulting product is not a simple aggregateof individual graphene sheets, but a single entity that is essentially anetwork of interconnected giant molecules with an essentially infinitemolecular weight. This may also be described as a graphene poly-crystal(with several grains, but typically no discernible, well-defined grainboundaries). All the constituent graphene planes are very large inlateral dimensions (length and width) and, if the HTT is sufficientlyhigh (e.g. >1,500° C. or much higher), these graphene planes areessentially bonded together with one another. The graphene foam of thepresently invented anode layer has the following unique and novelfeatures that have never been previously taught or hinted:

-   (1) In-depth studies using a combination of SEM, TEM, selected area    diffraction, X-ray diffraction, AFM, Raman spectroscopy, and FTIR    indicate that the graphene foam walls are composed of several huge    graphene planes (with length/width typically >>20 nm, more    typically >>100 nm, often >>1 μm, and, in many cases, >>10 μm, or    even >>100 μm). These giant graphene planes are stacked and bonded    along the thickness direction (crystallographic c-axis direction)    often through not just the van der Waals forces (as in conventional    graphite crystallites), but also covalent bonds, if the final heat    treatment temperature is lower than 2,500° C. In these cases,    wishing not to be limited by theory, but Raman and FTIR spectroscopy    studies appear to indicate the co-existence of sp² (dominating) and    sp³ (weak but existing) electronic configurations, not just the    conventional sp² in graphite.-   (2) This graphene foam wall is not made by gluing or bonding    discrete flakes/platelets together with a resin binder, linker, or    adhesive. Instead, GO sheets (molecules) from the GO dispersion or    the GF sheets from the GF dispersion are merged through joining or    forming of covalent bonds with one another, into an integrated    graphene entity, without using any externally added linker or binder    molecules or polymers. For a lithium battery featuring such an anode    layer, there is no need to have non-active materials, such as a    resin binder or a conductive additive, which are incapable of    storing lithium. This implies a reduced amount of non-active    materials or increased amount of active materials in the anode,    effectively increasing the specific capacity per total anode weight,    mAh/g (of composite).-   (3) The graphene foam walls are typically a poly-crystal composed of    large grains having incomplete grain boundaries. This entity is    derived from a GO or GF suspension, which is in turn obtained from    natural graphite or artificial graphite particles originally having    multiple graphite crystallites. Prior to being chemically oxidized    or fluorinated, these starting graphite crystallites have an initial    length (L_(a) in the crystallographic a-axis direction), initial    width (L_(b) in the b-axis direction), and thickness (L_(c) in the    c-axis direction). Upon oxidation or fluorination, these initially    discrete graphite particles are chemically transformed into highly    aromatic graphene oxide or graphene fluoride molecules having a    significant concentration of edge- or surface-borne functional    groups (e.g. —F, —OH, —COOH, etc.). These aromatic GO or GF    molecules in the suspension have lost their original identity of    being part of a graphite particle or flake. Upon removal of the    liquid component from the suspension, the resulting GO or GF    molecules form an essentially amorphous structure. Upon heat    treatments, these GO or GF molecules are chemically merged and    linked into a unitary or monolithic graphene entity that constitutes    the foam wall. This foam wall is highly ordered.

The resulting unitary graphene entity in the foam wall typically has alength or width significantly greater than the L_(a) and L_(b) of theoriginal crystallites. The length/width of this graphene foam wallentity is significantly greater than the L_(a) and L_(b) of the originalcrystallites. Even the individual grains in a poly-crystalline graphenewall structure have a length or width significantly greater than theL_(a) and L_(b) of the original crystallites.

-   (4) The large length and width of the graphene planes enable the    foam walls to be of high mechanical strength and elasticity. In    comparative experiments, we observe that without this feature (i.e.    no chemical merging of graphene planes), conventionally made    graphene foams composed of aggregates of discrete graphene sheets,    are very weak, fragile, and non-elastic (deformation not    reversible); foam walls being easily collapsed or broken.-   (5) Due to these unique chemical composition (including oxygen or    fluorine content), morphology, crystal structure (including    inter-graphene spacing), and structural features (e.g. high degree    of orientations, few defects, incomplete grain boundaries, chemical    bonding and no gap between graphene sheets, and substantially no    interruptions in graphene planes), the GO- or GF-derived graphene    foam has a unique combination of outstanding thermal conductivity,    electrical conductivity, mechanical strength, and stiffness (elastic    modulus).

The aforementioned features are further described and explained indetail as follows: As illustrated in FIG. 2(B), a graphite particle(e.g. 100) is typically composed of multiple graphite crystallites orgrains. A graphite crystallite is made up of layer planes of hexagonalnetworks of carbon atoms. These layer planes of hexagonally arrangedcarbon atoms are substantially flat and are oriented or ordered so as tobe substantially parallel and equidistant to one another in a particularcrystallite. These layers of hexagonal-structured carbon atoms, commonlyreferred to as graphene layers or basal planes, are weakly bondedtogether in their thickness direction (crystallographic c-axisdirection) by weak van der Waals forces and groups of these graphenelayers are arranged in crystallites. The graphite crystallite structureis usually characterized in terms of two axes or directions: the c-axisdirection and the a-axis (or b-axis) direction. The c-axis is thedirection perpendicular to the basal planes. The a- or b-axes are thedirections parallel to the basal planes (perpendicular to the c-axisdirection).

A highly ordered graphite particle can consist of crystallites of aconsiderable size, having a length of L_(a) along the crystallographica-axis direction, a width of L_(b) along the crystallographic b-axisdirection, and a thickness L_(c) along the crystallographic c-axisdirection. The constituent graphene planes of a crystallite are highlyaligned or oriented with respect to each other and, hence, theseanisotropic structures give rise to many properties that are highlydirectional. For instance, the thermal and electrical conductivity of acrystallite are of great magnitude along the plane directions (a- orb-axis directions), but relatively low in the perpendicular direction(c-axis). As illustrated in the upper-left portion of FIG. 2(B),different crystallites in a graphite particle are typically oriented indifferent directions and, hence, a particular property of amulti-crystallite graphite particle is the directional average value ofall the constituent crystallites.

Due to the weak van der Waals forces holding the parallel graphenelayers, natural graphite can be treated so that the spacing between thegraphene layers can be appreciably opened up so as to provide a markedexpansion in the c-axis direction, and thus form an expanded graphitestructure in which the laminar character of the carbon layers issubstantially retained. The process for manufacturing flexible graphiteis well-known in the art. In general, flakes of natural graphite (e.g.100 in FIG. 2(B)) are intercalated in an acid solution to producegraphite intercalation compounds (GICs, 102). The GICs are washed,dried, and then exfoliated by exposure to a high temperature for a shortperiod of time. This causes the flakes to expand or exfoliate in thec-axis direction of the graphite up to 80-300 times of their originaldimensions. The exfoliated graphite flakes are vermiform in appearanceand, hence, are commonly referred to as worms 104. These worms ofgraphite flakes which have been greatly expanded can be formed withoutthe use of a binder into cohesive or integrated sheets of expandedgraphite, e.g. webs, papers, strips, tapes, foils, mats or the like(typically referred to as “flexible graphite” 106) having a typicaldensity of about 0.04-2.0 g/cm³ for most applications.

The upper left portion of FIG. 2(A) shows a flow chart that illustratesthe prior art processes used to fabricate flexible graphite foils. Theprocesses typically begin with intercalating graphite particles 20(e.g., natural graphite or synthetic graphite) with an intercalant(typically a strong acid or acid mixture) to obtain a graphiteintercalation compound 22 (GIC). After rinsing in water to remove excessacid, the GIC becomes “expandable graphite.” The GIC or expandablegraphite is then exposed to a high temperature environment (e.g., in atube furnace preset at a temperature in the range of 800-1,050° C.) fora short duration of time (typically from 15 seconds to 2 minutes). Thisthermal treatment allows the graphite to expand in its c-axis directionby a factor of 30 to several hundreds to obtain a worm-like vermicularstructure 24 (graphite worm), which contains exfoliated, butun-separated graphite flakes with large pores interposed between theseinterconnected flakes.

In one prior art process, the exfoliated graphite (or mass of graphiteworms) is re-compressed by using a calendaring or roll-pressingtechnique to obtain flexible graphite foils (26 in FIG. 2(A) or 106 inFIG. 2(B)), which are typically 100-300 μm thick. In another prior artprocess, the exfoliated graphite worm 24 may be impregnated with a resinand then compressed and cured to form a flexible graphite composite,which is normally of low strength as well. In addition, upon resinimpregnation, the electrical and thermal conductivity of the graphiteworms could be reduced by two orders of magnitude.

Alternatively, the exfoliated graphite may be subjected tohigh-intensity mechanical shearing/separation treatments using ahigh-intensity air jet mill, high-intensity ball mill, or ultrasonicdevice to produce separated nano graphene platelets 33 (NGPs) with allthe graphene platelets thinner than 100 nm, mostly thinner than 10 nm,and, in many cases, being single-layer graphene (also illustrated as 112in FIG. 2(B)). An NGP is composed of a graphene sheet or a plurality ofgraphene sheets with each sheet being a two-dimensional, hexagonalstructure of carbon atoms. A mass of multiple NGPs (including discretesheets/platelets of single-layer and/or few-layer graphene or grapheneoxide, 33 in FIG. 2(A)) may be made into a graphene film/paper (34 inFIG. 2(A) or 114 in FIG. 2(B)) using a film- or paper-making process.

Further alternatively, with a low-intensity shearing, graphite wormstend to be separated into the so-called expanded graphite flakes (108 inFIG. 2(B) having a thickness >100 nm. These flakes can be formed intographite paper or mat 106 using a paper- or mat-making process. Thisexpanded graphite paper or mat 106 is just a simple aggregate or stackof discrete flakes having defects, interruptions, and mis-orientationsbetween these discrete flakes.

The following examples are used to illustrate some specific detailsabout the best modes of practicing the instant invention and should notbe construed as limiting the scope of the invention. For instance, weonly described the procedures use to produce graphene foam-protected Si,VO₂, Sn, SnO₂, and Co₃O₄ particles. But, any anode active material thatcan be made into fine particles (<20 μm in size) can be similarlyincorporated into a graphene suspension and made into graphenefoam-protected anode layer in a similar manner.

Example 1: Various Blowing Agents and Pore-Forming (Bubble-Producing)Processes

In the field of plastic processing, chemical blowing agents are mixedinto the plastic pellets in the form of powder or pellets and dissolvedat higher temperatures. Above a certain temperature specific for blowingagent dissolution, a gaseous reaction product (usually nitrogen or CO₂)is generated, which acts as a blowing agent. However, a chemical blowingagent cannot be dissolved in a graphene material, which is a solid, notliquid. This presents a challenge to make use of a chemical blowingagent to generate pores or cells in a graphene material.

After extensive experimenting, we have discovered that practically anychemical blowing agent (e.g. in a powder or pellet form) can be used tocreate pores or bubbles in a dried layer of graphene when the first heattreatment temperature is sufficient to activate the blowing reaction.The chemical blowing agent (powder or pellets) may be dispersed in theliquid medium to become a second dispersed phase (sheets of graphenematerial being the first dispersed phase) in the suspension, which canbe deposited onto the solid supporting substrate to form a wet layer.This wet layer of graphene material may then be dried and heat treatedto activate the chemical blowing agent. After a chemical blowing agentis activated and bubbles are generated, the resulting foamed graphenestructure is largely maintained even when subsequently a higher heattreatment temperature is applied to the structure. This is quiteunexpected, indeed.

Chemical foaming agents (CFAs) can be organic or inorganic compoundsthat release gasses upon thermal decomposition. CFAs are typically usedto obtain medium- to high-density foams, and are often used inconjunction with physical blowing agents to obtain low-density foams.CFAs can be categorized as either endothermic or exothermic, whichrefers to the type of decomposition they undergo. Endothermic typesabsorb energy and typically release carbon dioxide and moisture upondecomposition, while the exothermic types release energy and usuallygenerate nitrogen when decomposed. The overall gas yield and pressure ofgas released by exothermic foaming agents is often higher than that ofendothermic types. Endothermic CFAs are generally known to decompose inthe range of 130 to 230° C. (266-446° F.), while some of the more commonexothermic foaming agents decompose around 200° C. (392° F.). However,the decomposition range of most exothermic CFAs can be reduced byaddition of certain compounds. The activation (decomposition)temperatures of CFAs fall into the range of our heat treatmenttemperatures. Examples of suitable chemical blowing agents includesodium bicarbonate (baking soda), hydrazine, hydrazide, azodicarbonamide(exothermic chemical blowing agents), nitroso compounds (e.g. N,N-Dinitroso pentamethylene tetramine), hydrazine derivatives (e.g. 4.4′-Oxybis (benzenesulfonyl hydrazide) and Hydrazo dicarbonamide), andhydrogen carbonate (e.g. Sodium hydrogen carbonate). These are allcommercially available in plastics industry.

In the production of foamed plastics, physical blowing agents aremetered into the plastic melt during foam extrusion or injection moldedfoaming, or supplied to one of the precursor materials duringpolyurethane foaming. It has not been previously known that a physicalblowing agent can be used to create pores in a graphene material, whichis in a solid state (not melt). We have surprisingly observed that aphysical blowing agent (e.g. CO₂ or N₂) can be injected into the streamof graphene suspension prior to being coated or cast onto the supportingsubstrate. This would result in a foamed structure even when the liquidmedium (e.g. water and/or alcohol) is removed. The dried layer ofgraphene material is capable of maintaining a controlled amount of poresor bubbles during liquid removal and subsequent heat treatments.

Technically feasible blowing agents include Carbon dioxide (CO₂),Nitrogen (N₂), Isobutane (C₄H₁₀), Cyclopentane (C₅H₁₀), Isopentane(C₅H₁₂), CFC-11 (CFCI₃), HCFC-22 (CHF₂CI), HCFC-142b (CF₂CICH₃), andHCFC-134a (CH₂FCF₃). However, in selecting a blowing agent,environmental safety is a major factor to consider. The MontrealProtocol and its influence on consequential agreements pose a greatchallenge for the producers of foam. Despite the effective propertiesand easy handling of the formerly applied chlorofluorocarbons, there wasa worldwide agreement to ban these because of their ozone depletionpotential (ODP). Partially halogenated chlorofluorocarbons are also notenvironmentally safe and therefore already forbidden in many countries.The alternatives are hydrocarbons, such as isobutane and pentane, andthe gases such as CO₂ and nitrogen.

Except for those regulated substances, all the blowing agents recitedabove have been tested in our experiments. For both physical blowingagents and chemical blowing agents, the blowing agent amount introducedinto the suspension is defined as a blowing agent-to-graphene materialweight ratio, which is typically from 0/1.0 to 1.0/1.0.

Example 2: Preparation of Discrete Nano Graphene Platelets (NGPs) whichare GO Sheets

Chopped graphite fibers with an average diameter of 12 μm and naturalgraphite particles were separately used as a starting material, whichwas immersed in a mixture of concentrated sulfuric acid, nitric acid,and potassium permanganate (as the chemical intercalate and oxidizer) toprepare graphite intercalation compounds (GICs). The starting materialwas first dried in a vacuum oven for 24 h at 80° C. Then, a mixture ofconcentrated sulfuric acid, fuming nitric acid, and potassiumpermanganate (at a weight ratio of 4:1:0.05) was slowly added, underappropriate cooling and stirring, to a three-neck flask containing fibersegments. After 5-16 hours of reaction, the acid-treated graphite fibersor natural graphite particles were filtered and washed thoroughly withdeionized water until the pH level of the solution reached 6. Afterbeing dried at 100° C. overnight, the resulting graphite intercalationcompound (GIC) or graphite oxide fiber was re-dispersed in water and/oralcohol to form a slurry.

In one sample, five grams of the graphite oxide fibers were mixed with2,000 ml alcohol solution consisting of alcohol and distilled water witha ratio of 15:85 to obtain a slurry mass. Then, the mixture slurry wassubjected to ultrasonic irradiation with a power of 200 W for variouslengths of time. After 20 minutes of sonication, GO fibers wereeffectively exfoliated and separated into thin graphene oxide sheetswith oxygen content of approximately 23%-31% by weight. The resultingsuspension contains GO sheets being suspended in water. A chemicalblowing agent (hydrazo dicarbonamide) was added to the suspension justprior to casting.

The resulting suspension was then cast onto a glass surface using adoctor's blade to exert shear stresses, inducing GO sheet orientations.The resulting GO coating films, after removal of liquid, have athickness that can be varied from approximately 5 to 500 μm (preferablyand typically from 10 μm to 50 μm).

For making a graphene foam specimen, the GO coating film was thensubjected to heat treatments that typically involve an initial thermalreduction temperature of 80-350° C. for 1-8 hours, followed byheat-treating at a second temperature of 1,500-2,850° C. for 0.5 to 5hours. It may be noted that we have found it essential to apply acompressive stress to the coating film sample while being subjected tothe first heat treatment. This compress stress seems to have helpedmaintain good contacts between the graphene sheets so that chemicalmerging and linking between graphene sheets can occur while pores arebeing formed. Without such a compressive stress, the heat-treated filmis typically excessively porous with constituent graphene sheets in thepore walls being very poorly oriented and incapable of chemical mergingand linking with one another. As a result, the thermal conductivity,electrical conductivity, and mechanical strength of the graphene foamare severely compromised.

Example 3: Preparation of Single-Layer Graphene Sheets from Meso-CarbonMicro-Beads (MCMBs)

Meso-carbon microbeads (MCMBs) were supplied from China Steel ChemicalCo., Kaohsiung, Taiwan. This material has a density of about 2.24 g/cm³with a median particle size of about 16 μm. MCMB (10 grams) wereintercalated with an acid solution (sulfuric acid, nitric acid, andpotassium permanganate at a ratio of 4:1:0.05) for 48-96 hours. Uponcompletion of the reaction, the mixture was poured into deionized waterand filtered. The intercalated MCMBs were repeatedly washed in a 5%solution of HCl to remove most of the sulphate ions. The sample was thenwashed repeatedly with deionized water until the pH of the filtrate wasno less than 4.5. The slurry was then subjected ultrasonication for10-100 minutes to produce GO suspensions. TEM and atomic forcemicroscopic studies indicate that most of the GO sheets weresingle-layer graphene when the oxidation treatment exceeded 72 hours,and 2- or 3-layer graphene when the oxidation time was from 48 to 72hours.

The GO sheets contain oxygen proportion of approximately 35%-47% byweight for oxidation treatment times of 48-96 hours. GO sheets weresuspended in water. Baking soda (5-20% by weight), as a chemical blowingagent, was added to the suspension just prior to casting. The suspensionwas then cast onto a glass surface using a doctor's blade to exert shearstresses, inducing GO sheet orientations. Several samples were cast,some containing a blowing agent and some not. The resulting GO films,after removal of liquid, have a thickness that can be varied fromapproximately 10 to 500 μm.

The several sheets of the GO film, with or without a blowing agent, werethen subjected to heat treatments that involve an initial (first)thermal reduction temperature of 80-500° C. for 1-5 hours. This firstheat treatment generated a graphene foam. However, the graphene domainsin the foam wall can be further perfected (re-graphitized to become moreordered or having a higher degree of crystallinity and larger lateraldimensions of graphene planes, longer than the original graphene sheetdimensions due to chemical merging) if the foam is followed byheat-treating at a second temperature of 1,500-2,850° C.

Example 4: Preparation of Pristine Graphene Foam (0% Oxygen)

Recognizing the possibility of the high defect population in GO sheetsacting to reduce the conductivity of individual graphene plane, wedecided to study if the use of pristine graphene sheets (non-oxidizedand oxygen-free, non-halogenated and halogen-free, etc.) can lead to agraphene foam having a higher thermal conductivity. Pristine graphenesheets were produced by using the direct ultrasonication or liquid-phaseproduction process.

In a typical procedure, five grams of graphite flakes, ground toapproximately 20 μm or less in sizes, were dispersed in 1,000 mL ofdeionized water (containing 0.1% by weight of a dispersing agent, Zonyl®FSO from DuPont) to obtain a suspension. An ultrasonic energy level of85 W (Branson 5450 Ultrasonicator) was used for exfoliation, separation,and size reduction of graphene sheets for a period of 15 minutes to 2hours. The resulting graphene sheets are pristine graphene that havenever been oxidized and are oxygen-free and relatively defect-free.There are no other non-carbon elements.

Various amounts (1%-30% by weight relative to graphene material) ofchemical bowing agents (N, N-Dinitroso pentamethylene tetramine or 4.4′-Oxybis (benzenesulfonyl hydrazide) were added to a suspensioncontaining pristine graphene sheets and a surfactant. The suspension wasthen cast onto a glass surface using a doctor's blade to exert shearstresses, inducing graphene sheet orientations. Several samples werecast, including one that was made using CO₂ as a physical blowing agentintroduced into the suspension just prior to casting). The resultinggraphene films, after removal of liquid, have a thickness that can bevaried from approximately 10 to 100 μm.

The graphene films were then subjected to heat treatments that involvean initial (first) thermal reduction temperature of 80-1,500° C. for 1-5hours. This first heat treatment led to the production of a graphenefoam. Some of the pristine foam samples were then subjected to a secondtemperature of 1,500-2,850° C. to determine if the graphene domains inthe foam wall could be further perfected (re-graphitized to become moreordered or having a higher degree of crystallinity).

Example 4-a and Comparative Example 4-5: Pristine GrapheneFoam-Protected Anode vs. Prior Art Pristine GraphenePaper/Film-Protected Anode

Separately, a graphene film containing 65% by weight of Si particles(plus 5% by weight of the chemical blowing agent) was cast and heattreated up to 1,500° C. to obtain a layer of graphene foam protectedanode active material. For comparison purpose, a graphene film (paper)containing 65% by weight of Si particles (without any blowing agent) wascast and heat treated up to 1,500° C. to obtain a layer of grapheneprotected anode active material. The anode specific capacity of thesetwo anode layers was then evaluated using a lithium metal as thecounter-electrode in a half-cell configuration. The specific capacityvalues of a lithium battery having a pristine graphene foam-protected Siand those of a pristine graphene-Si mixture as an electrode material areplotted as a function of the number of charge-discharge cycles. Theseresults clearly demonstrate that the presently invented graphene foamhaving small pores, along with those pores occupied by Si particles (orSi particles protected by the wrapped around graphene sheets), provide alithium battery with more stable cycling stability, exhibiting only a7.0% reduction in lithium storage capacity (from 2450 to 2279 mAh/gbased on total anode composite weight) after 1,000 cycles. In contrast,the graphene film-protected Si anode exhibits a 24.9% capacity fade(from 2,448 to 1846 mAh/g).

Comparative Example 3/4-b: CVD Graphene Foams on Ni Foam Templates

The procedure was adapted from that disclosed in open literature: Chen,Z. et al. “Three-dimensional flexible and conductive interconnectedgraphene networks grown by chemical vapor deposition,” Nat. Mater. 10,424-428 (2011). Nickel foam, a porous structure with an interconnected3D scaffold of nickel was chosen as a template for the growth ofgraphene foam. Briefly, carbon was introduced into a nickel foam bydecomposing CH₄ at 1,000° C. under ambient pressure, and graphene filmswere then deposited on the surface of the nickel foam. Due to thedifference in the thermal expansion coefficients between nickel andgraphene, ripples and wrinkles were formed on the graphene films. Inorder to recover (separate) graphene foam, Ni frame must be etched away.Before etching away the nickel skeleton by a hot HCl (or FeCl₃)solution, a thin layer of poly(methyl methacrylate) (PMMA) was depositedon the surface of the graphene films as a support to prevent thegraphene network from collapsing during nickel etching. After the PMMAlayer was carefully removed by hot acetone, a fragile graphene foamsample was obtained. The use of the PMMA support layer is critical topreparing a free-standing film of graphene foam; only a severelydistorted and deformed graphene foam sample was obtained without thePMMA support layer. This is a tedious process that is notenvironmentally benign and is not scalable.

Comparative Example 3/4-c: Conventional Graphitic Foam from Pitch-BasedCarbon Foams

Pitch powder, granules, or pellets are placed in a aluminum mold withthe desired final shape of the foam. Mitsubishi ARA-24 meso-phase pitchwas utilized. The sample is evacuated to less than 1 torr and thenheated to a temperature approximately 300° C. At this point, the vacuumwas released to a nitrogen blanket and then a pressure of up to 1,000psi was applied. The temperature of the system was then raised to 800°C. This was performed at a rate of 2 degree C./min. The temperature washeld for at least 15 minutes to achieve a soak and then the furnacepower was turned off and cooled to room temperature at a rate ofapproximately 1.5 degree C./min with release of pressure at a rate ofapproximately 2 psi/min. Final foam temperatures were 630° C. and 800°C. During the cooling cycle, pressure is released gradually toatmospheric conditions. The foam was then heat treated to 1050° C.(carbonized) under a nitrogen blanket and then heat treated in separateruns in a graphite crucible to 2500° C. and 2800° C. (graphitized) inArgon.

Samples from this conventional graphitic foam were machined intospecimens for measuring the thermal conductivity. The bulk thermalconductivity of the graphitic foam was found to be in the range from 67W/mK to 151 W/mK. The density of the samples was from 0.31 to 0.61g/cm³. When the material porosity level is taken into account, thespecific thermal conductivity of the meso-phase pitch derived foam isapproximately 67/0.31=216 and 151/0.61=247.5 W/mK per specific gravity(or per physical density). In contrast, the specific thermalconductivity of the presently invented foam is typically >>250 W/mK perspecific gravity.

The compression strength of the conventional graphitic foam sampleshaving an average density of 0.51 g/cm³ was measured to be 3.6 MPa andthe compression modulus was measured to be 74 MPa. By contrast, thecompression strength and compressive modulus of the presently inventedgraphene foam samples derived from GO having a comparable physicaldensity are 5.7 MPa and 103 MPa, respectively.

Shown in FIG. 5(A) and FIG. 6(A) are the thermal conductivity values vs.specific gravity of the GO suspension-derived foam (Example 3),meso-phase pitch-derived graphite foam (Comparative Example 3/4-b), andNi foam template-assisted CVD graphene foam (Comparative Example 3/4-c).These data clearly demonstrate the following unexpected results:

-   -   1) GO-derived graphene foams produced by the presently invented        process exhibit significantly higher thermal conductivity as        compared to both meso-phase pitch-derived graphite foam and Ni        foam template-assisted CVD graphene, given the same physical        density.    -   2) This is quite surprising in view of the notion that CVD        graphene is essentially pristine graphene that has never been        exposed to oxidation and should have exhibited a much higher        thermal conductivity compared to graphene oxide (GO). GO is        known to be highly defective (having a high defect population        and, hence, low conductivity) even after the oxygen-containing        functional groups are removed via conventional thermal or        chemical reduction methods. These exceptionally high thermal        conductivity values observed with the GO-derived graphene foams        herein produced are much to our surprise. A good thermal        dissipation capability is essential to the prevention of thermal        run-away and explosion, a most serious problem associated with        rechargeable lithium-ion batteries.    -   3) FIG. 6(A) presents the thermal conductivity values over        comparable ranges of specific gravity values to allow for        calculation of specific conductivity (conductivity value, W/mK,        divided by physical density value, g/cm³) for all three        graphitic foam materials based on the slopes of the curves        (approximately straight lines at different segments). These        specific conductivity values enable a fair comparison of thermal        conductivity values of these three types of graphitic foams        given the same amount of solid graphitic material in each foam.        These data provide an index of the intrinsic conductivity of the        solid portion of the foam material. These data clearly indicate        that, given the same amount of solid material, the presently        invented GO-derived foam is intrinsically most conducting,        reflecting a high level of graphitic crystal perfection (larger        crystal dimensions, fewer grain boundaries and other defects,        better crystal orientation, etc.). This is also unexpected.    -   4) The specific conductivity values of the presently invented        GO- and GF-derived foam exhibit values from 250 to 500 W/mK per        unit of specific gravity; but those of the other two foam        materials are typically lower than 250 W/mK per unit of specific        gravity.

Summarized in FIG. 8 are thermal conductivity data for a series ofGO-derived graphene foams and a series of pristine graphene derivedfoams, both plotted over the final (maximum) heat treatmenttemperatures. These data indicate that the thermal conductivity of theGO foams is highly sensitive to the final heat treatment temperature(HTT). Even when the HTT is very low, clearly some type of graphenemerging or crystal perfection reactions are already activated. Thethermal conductivity increases monotonically with the final HTT. Incontrast, the thermal conductivity of pristine graphene foams remainsrelatively constant until a final HTT of approximately 2,500° C. isreached, signaling the beginning of a re-crystallization and perfectionof graphite crystals. There are no functional groups in pristinegraphene, such as —COOH in GO, that enable chemical linking of graphenesheets at relatively low HTTs. With a HTT as low as 1,250° C., GO sheetscan merge to form significantly larger graphene sheets with reducedgrain boundaries and other defects. Even though GO sheets areintrinsically more defective than pristine graphene, the presentlyinvented process enables the GO sheets to form graphene foams thatoutperform pristine graphene foams. This is another unexpected result.

Example 5: Preparation of Graphene Oxide (GO) Suspension from NaturalGraphite and Preparation of Subsequent GO Foams

Graphite oxide was prepared by oxidation of graphite flakes with anoxidizer liquid consisting of sulfuric acid, sodium nitrate, andpotassium permanganate at a ratio of 4:1:0.05 at 30° C. When naturalgraphite flakes (particle sizes of 14 μm) were immersed and dispersed inthe oxidizer mixture liquid for 48 hours, the suspension or slurryappears and remains optically opaque and dark. After 48 hours, thereacting mass was rinsed with water 3 times to adjust the pH value to atleast 3.0. A final amount of water was then added to prepare a series ofGO-water suspensions. We observed that GO sheets form a liquid crystalphase when GO sheets occupy a weight fraction >3% and typically from 5%to 15%.

By dispensing and coating the GO suspension on a polyethyleneterephthalate (PET) film in a slurry coater and removing the liquidmedium from the coated film we obtained a thin film of dried grapheneoxide. Several GO film samples were then subjected to different heattreatments, which typically include a thermal reduction treatment at afirst temperature of 100° C. to 500° C. for 1-10 hours, and at a secondtemperature of 1,500° C.-2,850° C. for 0.5-5 hours. With these heattreatments, also under a compressive stress, the GO films weretransformed into graphene foam.

Summarized in FIG. 6(C) are the specific capacities of several anodelayers for comparison purposes: the presently invented GO-derivedgraphene foam-protected Sn, Sn only (without graphene foam protection),graphene foam only, and theoretical prediction based on therule-of-mixture law. These data demonstrate an unexpected synergisticeffect between the presently invented GO-derived graphene foam and Snanode active material itself. It may be noted that graphene itself hassome lithium-storage capability. Even when this intrinsic lithiumstorage capacity of graphene is taken into consideration, thetheoretical prediction values are always significantly lower than thoseof the presently invented anode layer. This is a very significant andsurprising discovery.

Comparative Example 5-a: Graphene Foams from Hydrothermally ReducedGraphene Oxide

For comparison, a self-assembled graphene hydrogel (SGH) sample wasprepared by a one-step hydrothermal method. In a typical procedure, theSGH can be easily prepared by heating 2 mg/mL of homogeneous grapheneoxide (GO) aqueous dispersion sealed in a Teflon-lined autoclave at 180°C. for 12 h. The SGH containing about 2.6% (by weight) graphene sheetsand 97.4% water has an electrical conductivity of approximately 5×10⁻³S/cm. Upon drying and heat treating at 1,500° C., the resulting graphenefoam exhibits an electrical conductivity of approximately 1.5×10⁻¹ S/cm,which is 2 times lower than those of the presently invented graphenefoams produced by heat treating at the same temperature.

FIG. 6(D) shows the specific capacities of two anode layers: thepresently invented GO-derived graphene foam-protected SnO₂ andhydrothermally reduced GO-derived graphene foam-protected SnO₂. Clearly,the presently invented GO-derived graphene foam is more effective inprotecting SnO₂ particles possibly due to the more elastic and higherstrength of the presently invented graphene foam walls as compared tothe process produced by the hydrothermal reduction. This is againunexpected.

Comparative Example 5-b: Plastic Bead Template-Assisted Formation ofReduced Graphene Oxide Foams

A hard template-directed ordered assembly for a macro-porous bubbledgraphene film (MGF) was prepared. Mono-disperse poly methyl methacrylate(PMMA) latex spheres were used as the hard templates. The GO liquidcrystal prepared in Example 5 was mixed with a PMMA spheres suspension.Subsequent vacuum filtration was then conducted to prepare the assemblyof PMMA spheres and GO sheets, with GO sheets wrapped around the PMMAbeads. A composite film was peeled off from the filter, air dried andcalcinated at 800° C. to remove the PMMA template and thermally reduceGO into RGO simultaneously. The grey free-standing PMMA/GO film turnedblack after calcination, while the graphene film remained porous.

FIG. 5(B) and FIG. 6(B) show the thermal conductivity values of thepresently invented GO suspension-derived foam, GO foam produced viasacrificial plastic bead template-assisted process, and hydrothermallyreduced GO graphene foam. Most surprisingly, given the same starting GOsheets, the presently invented process produces the highest-performinggraphene foams. Electrical conductivity data summarized in FIG. 4(C) arealso consistent with this conclusion. These data further support thenotion that, given the same amount of solid material, the presentlyinvented GO suspension deposition (with stress-induced orientation) andsubsequent heat treatments give rise to a graphene foam that isintrinsically most conducting, reflecting a highest level of graphiticcrystal perfection (larger crystal dimensions, fewer grain boundariesand other defects, better crystal orientation, etc. along the porewalls).

It is of significance to point out that all the prior art processes forproducing graphite foams or graphene foams appear to providemacro-porous foams having a physical density in the range ofapproximately 0.2-0.6 g/cm³ only with pore sizes being typically toolarge (e.g. from 20 to 300 μm) for most of the intended applications. Incontrast, the instant invention provides processes that generategraphene foams having a density that can be as low as 0.01 g/cm³ and ashigh as 1.7 g/cm³. The pore sizes can be varied between meso-scaled(2-50 nm) up to macro-scaled (1-500 μm) depending upon the contents ofnon-carbon elements and the amount/type of blowing agent used. Thislevel of flexibility and versatility in designing various types ofgraphene foams is unprecedented and un-matched by any prior art process.

Example 6: Preparation of Graphene Foams from Graphene Fluoride

Several processes have been used by us to produce GF, but only oneprocess is herein described as an example. In a typical procedure,highly exfoliated graphite (HEG) was prepared from intercalated compoundC₂F.xClF₃. HEG was further fluorinated by vapors of chlorine trifluorideto yield fluorinated highly exfoliated graphite (FHEG). Pre-cooledTeflon reactor was filled with 20-30 mL of liquid pre-cooled ClF₃, thereactor was closed and cooled to liquid nitrogen temperature. Then, nomore than 1 g of HEG was put in a container with holes for ClF₃ gas toaccess and situated inside the reactor. In 7-10 days a gray-beigeproduct with approximate formula C₂F was formed.

Subsequently, a small amount of FHEG (approximately 0.5 mg) was mixedwith 20-30 mL of an organic solvent (methanol, ethanol, 1-propanol,2-propanol, 1-butanol, tert-butanol, isoamyl alcohol) and subjected toan ultrasound treatment (280 W) for 30 min, leading to the formation ofhomogeneous yellowish dispersions. Five minutes of sonication was enoughto obtain a relatively homogenous dispersion, but longer sonicationtimes ensured better stability. Upon casting on a glass surface with thesolvent removed, the dispersion became a brownish film formed on theglass surface. When GF films were heat-treated, fluorine was released asgases that helped to generate pores in the film. In some samples, aphysical blowing agent (N₂ gas) was injected into the wet GF film whilebeing cast. These samples exhibit much higher pore volumes or lower foamdensities. Without using a blowing agent, the resulting graphenefluoride foams exhibit physical densities from 0.35 to 1.38 g/cm³. Whena blowing agent was used (blowing agent/GF weight ratio from 0.5/1 to0.05/1), a density from 0.02 to 0.35 g/cm³ was obtained. Typicalfluorine contents are from 0.001% (HTT=2,500° C.) to 4.7% (HTT=350° C.),depending upon the final heat treatment temperature involved.

FIG. 6 presents a comparison in thermal conductivity values of thegraphene foam samples derived from GO and GF (graphene fluoride),respectively, as a function of the specific gravity. It appears that theGF foams, in comparison with GO foams, exhibit higher thermalconductivity values at comparable specific gravity values. Both deliverimpressive heat-conducting capabilities, being the best among all knownfoamed materials.

Example 7: Preparation of Graphene Foams from Nitrogenataed Graphene

Graphene oxide (GO), synthesized in Example 2, was finely ground withdifferent proportions of urea and the pelletized mixture heated in amicrowave reactor (900 W) for 30 s. The product was washed several timeswith deionized water and vacuum dried. In this method graphene oxidegets simultaneously reduced and doped with nitrogen. The productsobtained with graphene:urea mass ratios of 1:0.5, 1:1 and 1:2 aredesignated as NGO-1, NGO-2 and NGO-3 respectively and the nitrogencontents of these samples were 14.7, 18.2 and 17.5 wt % respectively asfound by elemental analysis. These nitrogenataed graphene sheets remaindispersible in water. The resulting suspensions were then cast, dried,and heat-treated initially at 200-350° C. as a first heat treatmenttemperature and subsequently treated at a second temperature of 1,500°C. The resulting nitrogenated graphene foams exhibit physical densitiesfrom 0.45 to 1.28 g/cm³. Typical nitrogen contents of the foams are from0.01% (HTT=1,500° C.) to 5.3% (HTT=350° C.), depending upon the finalheat treatment temperature involved.

Example 8: Characterization of Various Graphene Foams and ConventionalGraphite Foam

The internal structures (crystal structure and orientation) of severaldried GO layers, and the heat-treated films at different stages of heattreatments were investigated using X-ray diffraction. The X-raydiffraction curve of natural graphite typically exhibits a peak atapproximately 2θ=26°, corresponds to an inter-graphene spacing (d₀₀₂) ofapproximately 0.3345 nm. Upon oxidation, the resulting GO shows an X-raydiffraction peak at approximately 2θ=12°, which corresponds to aninter-graphene spacing (d₀₀₂) of approximately 0.7 nm. With some heattreatment at 150° C., the dried GO compact exhibits the formation of ahump centered at 22°, indicating that it has begun the process ofdecreasing the inter-graphene spacing due to the beginning of chemicallinking and ordering processes. With a heat treatment temperature of2,500° C. for one hour, the d₀₀₂ spacing has decreased to approximately0.336, close to 0.3354 nm of a graphite single crystal.

With a heat treatment temperature of 2,750° C. for one hour, the d₀₀₂spacing is decreased to approximately to 0.3354 nm, identical to that ofa graphite single crystal. In addition, a second diffraction peak with ahigh intensity appears at 2θ=55° corresponding to X-ray diffraction from(004) plane. The (004) peak intensity relative to the (002) intensity onthe same diffraction curve, or the I(004)/I(002) ratio, is a goodindication of the degree of crystal perfection and preferred orientationof graphene planes. The (004) peak is either non-existing or relativelyweak, with the I(004)/I(002) ratio<0.1, for all graphitic materials heattreated at a temperature lower than 2,800° C. The I(004)/I(002) ratiofor the graphitic materials heat treated at 3,000-3,250° C. (e,g, highlyoriented pyrolytic graphite, HOPG) is in the range of 0.2-0.5. Incontrast, a graphene foam prepared with a final HTT of 2,750° C. for onehour exhibits a I(004)/I(002) ratio of 0.78 and a Mosaic spread value of0.21, indicating a practically perfect graphene single crystal with agood degree of preferred orientation.

The “mosaic spread” value is obtained from the full width at halfmaximum of the (002) reflection in an X-ray diffraction intensity curve.This index for the degree of ordering characterizes the graphite orgraphene crystal size (or grain size), amounts of grain boundaries andother defects, and the degree of preferred grain orientation. A nearlyperfect single crystal of graphite is characterized by having a mosaicspread value of 0.2-0.4. Some of our graphene foams have a mosaic spreadvalue in this range of 0.2-0.4 when produced using a final heattreatment temperature no less than 2,500° C.

The inter-graphene spacing values of both the GO suspension-derivedsamples obtained by heat treating at various temperatures over a widetemperature range are summarized in FIG. 9(A). Corresponding oxygencontent values in the GO suspension-derived unitary graphene layer areshown in FIG. 9(B).

It is of significance to point out that a heat treatment temperature aslow as 500° C. is sufficient to bring the average inter-graphene spacingin GO sheets along the pore walls to below 0.4 nm, getting closer andcloser to that of natural graphite or that of a graphite single crystal.The beauty of this approach is the notion that this GO suspensionstrategy has enabled us to re-organize, re-orient, and chemically mergethe planar graphene oxide molecules from originally different graphiteparticles or graphene sheets into a unified structure with all thegraphene planes now being larger in lateral dimensions (significantlylarger than the length and width of the graphene planes in the originalgraphite particles). A potential chemical linking mechanism isillustrated in FIG. 4. This has given rise to exceptional thermalconductivity and electrical conductivity values.

Example 9: Cycle Stability of Various Rechargeable Lithium Battery Cells

In lithium-ion battery industry, it is a common practice to define thecycle life of a battery as the number of charge-discharge cycles thatthe battery suffers 20% decay in capacity based on the initial capacitymeasured after the required electrochemical formation. Summarized inTable 1 below are the cycle life data of a broad array of batteriesfeaturing a presently invented graphene foam-protected anode layer vs.other types of anode materials.

TABLE 1 Cycle life data of various lithium secondary (rechargeable)batteries (ρ = physical density). Protective material (type of grapheneInitial Cycle life foam or conductive Type & % of anode capacity (No. ofSample ID additive/binder) active material (mAh/g) cycles) CommentsSi-GO-3 GO foam (HTT = 25% by wt. Si nano 1,250 755-1,275 Longest life1,510° C.; ρ = 0.32- particles (80 nm) when ρ = 1.68 g/cm³) 0.6-1.5g/cm³ Si-comp 67% graphite + 8% 25% by wt. Si nano 1,242 454 No graphenebinder particles (80 nm) Si-GF-4 Graphene fluoride 35% Si nanowires1,388 745 (with Cycle life foam (HTT = (diameter = 90 nm) empty 347(without 2520° C.) ρ = 0.7 small pores) particle-free g/cm³ pores)Si-NG-5 Nitrogenated 45% Si nano 1,852 1,420 (pre); ρ = 0.87 graphenefoam particles, prelithiated 677 (non-pre) g/cm³ or non-prelithiatedVO₂-G-6 Pristine graphene 90%-95%, VO₂ 240-280 1677  foam nano ribbonCo₃O₄-BG- B-doped or non- 85% Co₃O₄ 720 832 (B- 7 doped graphene doped);foam 576 (no B) SnO₂-8 RGO foam; 75% SnO₂ particles 770 788 165 cycles;compressed at (3 μm initial size) no 2,500° C. compression

These data further confirms:

-   -   (1) The graphene foam is very effective in alleviating the anode        expansion/shrinkage problems.    -   (2) Graphene foam containing pores that are not occupied by        anode active material particles (particle-free pores) are        significantly more effective in enhancing the cycle stability of        a lithium battery.    -   (3) Boron-doped graphene is more effective than non-doped        graphene as an anode protector.    -   (4) The issue of larger anode particles (μm size) having a        higher tendency to get pulverized can be addressed by heating        and compressing the graphene-wrapped anode particles in such a        manner that the anode material gets melted and the liquid-like        anode material permeates into minute pores to form a tree-like        anode material structure with largely nanometer-scaled branches.        These nano-structured anode active materials protected by        graphene foam surprisingly provide the battery with a much        longer cycle life.

In conclusion, we have successfully developed an absolutely new, novel,unexpected, and patently distinct class of highly conducting graphenefoam-protected anode active materials and related processes ofproduction. The chemical composition (% of oxygen, fluorine, and othernon-carbon elements), structure (crystal perfection, grain size, defectpopulation, etc), crystal orientation, morphology, process ofproduction, and properties of this new class of graphene foam materialsand their protected anode layers are fundamentally different andpatently distinct from those of meso-phase pitch-derived graphite foam,CVD graphene-derived foam, anode protected by graphene foams fromhydrothermal reduction of GO, anode protected by sacrificial beadtemplate-assisted RGO foam, and solid graphene film/paper-protectedanode. The presently invented foam materials provide better thermalconductivity, electrical conductivity, elastic modulus, flexuralstrength, and anode-protecting capability as compared to any prior artfoam materials or non-foam materials.

We claim:
 1. A process for producing an anode layer for a lithium ionbattery, said process comprising: (a) preparing a graphene dispersionhaving multiple particles of anode active material and multiple sheetsof a starting graphene material dispersed in a liquid medium, whereinsaid starting graphene material comprises pristine graphene material ornon-pristine graphene material, where non-pristine is defined as havinga content of non-carbon elements greater than 2% by weight, and wheresaid starting graphene material is selected from the group consisting ofgraphene oxide, reduced graphene oxide, graphene fluoride, graphenechloride, graphene bromide, graphene iodide, hydrogenated graphene,nitrogenated graphene, chemically functionalized graphene, andcombinations thereof, and wherein said dispersion contains a blowingagent; (b) dispensing said graphene dispersion onto a surface of asupporting substrate to form a wet layer of graphene and anode activematerial mixture, wherein said dispensing procedure includes subjectingsaid graphene dispersion to an orientation-inducing stress; (c)partially or completely removing said liquid medium from the wet layerof graphene and anode active material to form a dried layer of mixturematerial; and (d) heat treating the dried layer of mixture material at afirst heat treatment temperature selected from 80° C. to 3,200° C. at adesired heating rate sufficient to induce volatile gas molecules fromsaid non-carbon elements or to activate said blowing agent for producinga pore-containing graphene foam anode layer.
 2. The process of claim 1,further including a step of heat-treating the graphene foam anode layerat a second heat treatment temperature higher than said first heattreatment temperature for a length of time sufficient for obtaining saidanode layer wherein said pores of the pore-containing graphene foamanode layer have walls that contain stacked graphene planes having aninter-plane spacing d₀₀₂ from 0.3354 nm to 0.36 nm and a content ofnon-carbon elements less than 2% by weight.
 3. The process of claim 1,wherein said blowing agent having a blowing agent-to-graphene weightratio from 0.01/1.0 to 1.0/1.0.
 4. The process of claim 1, wherein saidblowing agent is at least one of a physical blowing agent, a chemicalblowing agent, a mixture thereof, a dissolution-and-leaching agent, or amechanically introduced blowing agent.
 5. The process of claim 1, whichis a roll-to-roll process wherein said steps (b) and (c) include feedingsaid supporting substrate from a feeder roller to a deposition zone,continuously or intermittently dispensing or depositing said graphenedispersion onto a surface of said supporting substrate to form said wetlayer of graphene material thereon, drying said wet layer of graphenematerial to form the dried layer of graphene material, and collectingsaid dried layer of graphene material deposited on said supportingsubstrate on a collector roller.
 6. The process of claim 1, wherein saidfirst heat treatment temperature is in a range from 100° C. to 1,500° C.7. The process of claim 2, wherein said second heat treatmenttemperature includes a temperature selected from at least one of: (A)300-1,500° C., (B) 1,500-2,100° C., or (C) 2,100-3,200° C.
 8. Theprocess of claim 1, wherein said step (d) of heat treating the driedlayer of mixture material is conducted under a compressive stress. 9.The process of claim 1, wherein said liquid medium is an oxidizingliquid medium, wherein said graphene dispersion contains graphene oxideand the dispersion is prepared by immersing said starting graphenematerial in a powder or fibrous form in the liquid medium in a reactionvessel at a reaction temperature for a length of time sufficient toobtain said graphene dispersion and wherein said graphene oxide has anoxygen content no less than 5% by weight.
 10. The process of claim 1,wherein said anode active material is selected from the group consistingof: (a) silicon (Si), germanium (Ge), tin (Sn), lead (Pb), antimony(Sb), bismuth (Bi), zinc (Zn), aluminum (Al), titanium (Ti), nickel(Ni), cobalt (Co), or cadmium (Cd); (b) alloys or intermetalliccompounds of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Ti, Ni, Co, or Cd withother elements; (c) oxides, carbides, nitrides, sulfides, phosphides,selenides, and tellurides of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Ti, Fe, Ni,Co, V, or Cd, and their mixtures, composites, or lithium-containingcomposites; (d) salts or hydroxides of Sn; (e) lithium titanate, lithiummanganate, lithium aluminate, lithium-containing titanium oxide, orlithium transition metal oxide; (f) prelithiated versions thereof; (g)particles of Li, Li alloy, or surface-stabilized Li; and (h)combinations thereof.
 11. The process of claim 1, wherein said anodeactive material contains at least one of: prelithiated Si, prelithiatedGe, prelithiated Sn, prelithiated SnO_(x), prelithiated SiO_(x),prelithiated iron oxide, prelithiated VO₂, prelithiated Co₃O₄,prelithiated Ni₃O₄, or a combination thereof, wherein 1<x<2.
 12. Theprocess of claim 1, wherein said anode active material is in at leastone of the forms of: nanoparticle, nanowire, nanofiber, nanotube,nanosheet, nanobelt, nanoribbon, or nano-coating having a thickness ordiameter less than 100 nm.
 13. The process of claim 1, furthercomprising a lithium-conducting coating deposited onto said anode activematerial.
 14. The process of claim 1, wherein said anode active materialhas a dimension less than 20 nm.
 15. The process of claim 1, furthercomprising a carbon or graphite material in said dispersion, whereinsaid carbon or graphite material is in electronic contact with ordeposited onto said anode active material.
 16. The process of claim 15,wherein said carbon or graphite material is selected from the groupconsisting of: polymeric carbon, amorphous carbon, chemical vapordeposition carbon, coal tar pitch, petroleum pitch, mesophase pitch,carbon black, coke, acetylene black, activated carbon, fine expandedgraphite particle with a dimension smaller than 100 nm, artificialgraphite particle, natural graphite particle, and combinations thereof.17. The process of claim 15, further comprising a conductive protectivecoating, selected from the group consisting of a carbon material,electronically conductive polymer, conductive metal oxide, conductivemetal coating, and a lithium-conducting material, which then saidconductive protective coating is deposited onto or wrapped around saidactive anode material.
 18. The process of claim 1, wherein said graphenefoam anode layer contains pristine graphene and said anode layer has adensity from 0.5 to 1.7 g/cm³ or a pore size from 2 nm to 100 nm.
 19. Aroll-to-roll process for producing a continuous-length sheet of saidanode layer of claim 1, said process comprising: (a) preparing saidgraphene dispersion having said starting graphene material and saidanode active material dispersed in said liquid medium, wherein saiddispersion contains said blowing agent; (b) continuously orintermittently dispensing and depositing said graphene dispersion onto asurface of said supporting substrate to form said wet layer ofgraphene-anode material mixture, wherein said supporting substrate is acontinuous thin film supplied from a feeder roller and collected on acollector roller; (c) partially or completely removing said liquidmedium from said wet layer of graphene and anode active material mixtureto form said dried layer of mixture material; and (d) heat treating saiddried layer of mixture material at said first heat treatment temperaturein the range from 100° C. to 3,000° C. at said desired heating ratesufficient to activate said blowing agent for producing said graphenefoam anode layer.
 20. The process of claim 1, which is in acontinuous-length roll sheet form having a thickness no greater than 300μm and a length of at least 2 meters and is produced by a roll-to-rollprocess.
 21. The process of claim 1, wherein said anode layer has atleast one of: a density from 0.01 to 1.7 g/cm³, a specific surface areafrom 50 to 2,000 m²/g, a thermal conductivity of at least 100 W/mK perunit of specific gravity, or an electrical conductivity no less than1,000 S/cm per unit of specific gravity.
 22. The process of claim 1,wherein said anode layer has an oxygen content or non-carbon contentless than 1% by weight, and said pores of the pore-containing graphenefoam anode layer have walls that have at least one of: an inter-graphenespacing less than 0.35 nm, a thermal conductivity of at least 250 W/mKper unit of specific gravity, or an electrical conductivity no less than2,500 S/cm per unit of specific gravity.
 23. The process of claim 1,wherein said anode layer has an oxygen content or non-carbon contentless than 0.01% by weight and said pores of the pore-containing graphenefoam anode layer have walls that have at least one of: stacked grapheneplanes having an inter-graphene spacing less than 0.34 nm, a thermalconductivity of at least 300 W/mK per unit of specific gravity, or anelectrical conductivity no less than 3,000 S/cm per unit of specificgravity.
 24. The process of claim 1, wherein said anode layer has anoxygen content or non-carbon content no greater than 0.01% by weight andsaid pores of the pore-containing graphene foam anode layer have wallsthat have at least one of: stacked graphene planes having aninter-graphene spacing less than 0.336 nm, a mosaic spread value nogreater than 0.7, a thermal conductivity of at least 350 W/mK per unitof specific gravity, or an electrical conductivity no less than 3,500S/cm per unit of specific gravity.
 25. The process of claim 1, whereinsaid anode layer has pores of the pore-containing graphene foam anodelayer that have walls that have at least one of: stacked graphene planeshaving an inter-graphene spacing less than 0.336 nm, a mosaic spreadvalue no greater than 0.4, a thermal conductivity greater than 400 W/mKper unit of specific gravity, or an electrical conductivity greater than4,000 S/cm per unit of specific gravity.
 26. The process of claim 1,wherein said pores of the pore-containing graphene foam anode layer havewalls that include at least one of: stacked graphene planes having aninter-graphene spacing less than 0.337 nm or a mosaic spread value lessthan 1.0.
 27. The process of claim 1, wherein said anode layer has atleast one of: a degree of graphitization no less than 80% or a mosaicspread value less than 0.4.
 28. The process of claim 1, wherein saidpores of the pore-containing graphene foam anode layer have walls thatcontain a 3D network of interconnected graphene planes.
 29. The processof claim 1, wherein said anode layer contains pores having a pore sizein the range from 20 nm to 500 nm.